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Catalytic Fe3+ Clusters and Complexes in Nafion Active in Photo-Fenton Processes. High-Resolution Electron Microscopy and Femtosecond Studies J. Kiwi,*,† N. Denisov,‡ Y. Gak,‡ N. Ovanesyan,‡ P. A. Buffat,§ E. Suvorova,§ F. Gostev,⊥ A. Titov,⊥ O. Sarkisov,⊥ P. Albers,| and V. Nadtochenko*,‡ Laboratory of Photonics and Interfaces, Department of Chemistry, Swiss Federal Institute of Technology, 1015 Lausanne, Switzerland, Institute of Problems in Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka Moscow Region, Russian Federation, Institute of Electron Microscopy, Swiss Federal Institute of Technology, 1015 Lausanne, Switzerland, Infracor GmbH, Abt ATS-PCP, D-63403, Hanau/Main, Germany, and Institute of Chemical Physics, Russian Academy of Sciences, 117977, Kosigin St 4, Moscow, Russian Federation Received July 17, 2002 This study presents the detailed nature of iron clusters formed on Fe3+-Nafion membranes. The catalytic nature of these clusters during immobilized Fenton processes was observed to be a function of the deposition method of Fe ions on the Nafion. The nonbiodegradable azo-dye Orange II and 2-propanol were utilized as convenient organic model compounds in photoassisted Fenton degradation processes. The highest photocatalytic activity was observed when samples were prepared by ion exchange between iron(III) aquacomplexes and H+ or Na+ as counterions of the Nafion SO3- group. Spectroscopic techniques show that iron(III) in the membrane was present mainly as a mononuclear complex of [Fe(H2O)6]3+ and binuclear complexes [Fe(H3O2)Fe]5+ and [Fe-O-Fe]4+. If NaOH or ammonia was added to the former samples prepared by ion exchange, Nafion-Fe membranes with low photocatalytic activity were obtained showing R-Fe2O3 and [Fe-O-Fe]4+. Detailed high-resolution transmission electron microscopy was carried out for the Nafion-Fe ion-exchanged and also base-treated membranes showing R-Fe2O3 nanocrystallites of 3.5-5 nm. Spectral bands were found for iron oxides in the Fe3+-Nafion by femtosecond laser spectroscopy. The R-Fe2O3 nanocrystallites in the Nafion exchanged base-treated membranes presented a relaxation dynamics for the excited states close to that observed with R-Fe2O3 nanocrystallite colloids taken as reference compounds. Multiexponential transient absorption decay of R-Fe2O3 in SO3--water clusters was observed with time constants close to 320 fs, 1.5 ps, and 31 ps after the excitation pulse. Samples of Fe3+-Nafion membranes with high activity show different transient dynamics relative to the Fe3+-Nafion with low activity. Correlation of the photocatalytic activity of Fe3+-Nafion with UV-vis, Fourier transform infrared, Mo¨ssbauer, and X-ray photoelectron spectroscopic results suggests that the photocatalytic activity correlates with the amount of mononuclear [Fe(H2O)6 ]3+, binuclear complexes [Fe(H3O2)Fe]5+ and oxo-bridged [FeO-Fe]4+ found in the membranes.
Introduction During the past decade, the abatement of contaminants in aqueous solution activated by UV-vis light and in the presence of the Fenton reagent Fe2+/Fe3+/H2O2 has gained in importance and application.1-8 Heterogeneous photoassisted Fenton systems have been reported to proceed † Laboratory of Photonics and Interfaces, Swiss Federal Institute of Technology. ‡ Institute of Problems in Chemical Physics, Russian Academy of Sciences. § Institute of Electron Microscopy, Swiss Federal Institute of Technology. | Infracor GmbH. ⊥ Institute of Chemical Physics, Russian Academy of Sciences.
(1) Kiwi, J.; Lopez, A.; Nadtochenko, V. Environ. Sci. Technol. 2000, 34, 2162. (2) Nadtochenko, V.; Kiwi, J. Chem. Commun. 1997, 41. (3) Ruppert, G.; Hofstadler, K.; Bauer, R.; Heisler, G. Proc. Indian Acad. Sci. (Chem. Sci.) 1993, 105, 393. (4) Safarzadeh Amiri, A.; Bolton, J. R.; Cater, S. R. Solar Energy 1996, 56, 439. (5) Sapach, R.; Viraraghavan, T. J. Environ. Sci. Health, Part A 1997, 32, 2355. (6) Sun, Y. F.; Pignatello, J. J. Environ. Sci. Technol. 1993, 27, 304. (7) Yang, M.; Hu, J.; Ito, K. Environ. Technol. 1998, 19, 183. (8) Safarzadeh Amiri, A.; Bolton, J. R.; Cater, S. R. Water Res. 1997, 31, 787.
with a slower kinetics when compared with the homogeneous systems during the degradation of the organic pollutants.9-11 The activity of the Fenton catalytic system strongly depends on the nature of the iron complexes.11 Therefore, the acceleration of photocatalytic Fenton mediated reactions is a topical research subject. Recently, Fe ions have been immobilized on Nafion membranes, avoiding the costly Fe separation at the end of the process as required in homogeneous media.12,13 Until now, the detailed identification of the Fe species and their activity in photoassisted degradation processes has not been reported. The main goal of the present study is to correlate the Fe complexes and oxides in the Nafion membranes prepared by different procedures with catalytic activity of the loaded membranes during the degradation of some model organic compounds such as azo-dye Orange II and 2-propanol (IPA). The degradation of the model compounds (9) Allian, M.; Germain, A.; Figueras, F. Sci. Technol. Catal. 1994 1995, 92, 239. (10) Huang, H. H.; Lu, M. C.; Chen, J. N. Water Res. 2001, 35, 2291. (11) Sychiov, A. Y.; Isac, V. G. Uspekhi Khimii 1995, 64, 1183. (12) Fernandez, J.; Bandara, J.; Lopez, A.; Albers, P.; Kiwi, J. Chem. Commun. 1998, 1493. (13) Fernandez, J.; Bandara, J.; Lopez, A.; Buffat, P.; Kiwi, J. Langmuir 1999, 15, 185.
10.1021/la020648k CCC: $22.00 © 2002 American Chemical Society Published on Web 10/12/2002
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was observed to be related to the preparation and structural properties of the Fe3+-Nafion membranes. The nature of the active catalytic sites on the Fe3+-Nafion membranes in the presence of H2O2 will be elucidated by various spectroscopic techniques such as UV-vis, Fourier transform infrared (FTIR), X-ray photoelectron, and Mo¨ssbauer spectroscopy. By femtosecond laser photolysis and high-resolution transmission electron microscopy (HRTEM), we will show the effect of morphology and phase of Fe clusters and oxides in Nafion-SO3- on the degradation dynamics of two model organic compounds. The combination of the various spectroscopic techniques allows a detailed identification of the active iron complexes participating in the photocatalysis taking place and allows us to work out results that have not been reported in the scientific literature. Experimental Section Catalyst Preparation. Cation Nafion membrane 117 was obtained from Aldrich. Fe(ClO4)3‚9H2O, FeCl3‚6H2O, and Fe2(SO4)3‚aq were Fluka p.a. and used as received. Nafion membranes were washed with a mixture of H2O2/H2SO4 and finally with deionized water to clean the surface from organic contaminants. The H+-Nafion samples with 5 cm2 surface were simply equilibrated during their preparation with deionized water. Preparation of 5 cm2 Na+-Nafion membranes needed equilibration with solutions of 1 M NaOH (20 mL) overnight under constant stirring. Subsequently the membranes were washed thoroughly with deionized water. The Fe3+-Nafion membranes were prepared in three different ways: (1) The Nafion membrane (H+-Nafion) was dipped in diverse concentrations (0.05-0.28 mM) of freshly prepared Fe3+ solutions (25 mL), previously stabilized for 12 h to allow the ion exchange between H+ and Fe3+ leading to yellow Nafion membranes. (2) The Na+Nafion was used as above but in this case Na+ was exchanged by Fe3+ instead of H+ by Fe3+, also leading to yellow Nafion membranes. (3) After the ion exchange between H+ or Na+ in the Nafion with Fe3+, the Fe3+-Nafion membrane was immersed in NaOH (1 M) or ammonia (1 M) solution. After the addition of base, the Fe3+-Nafion color changed from yellow to brown. The Nafion membrane after base treatment will be called “brown” and without basic treatment will be called “yellow” from now on. Nanocrystallites of R-Fe2O3 were prepared by the dropwise addition of 50 mM FeCl3‚6H2O Fluka p.a. solution into boiling distilled water until a dilution of 1:20 was attained.14 A deep-red opalescent colored solution was formed, with R-Fe2O3 nanoparticles having a particle size distribution of 2-25 nm as observed by HRTEM. Photocatalytic Efficiency of the Fe3+-Nafion Membranes. Irradiation of the Orange II solutions in cylindrical reactors (Pyrex flasks, 40 mL of reagent solution) was carried out inside the cavity of a Hanau Suntest (290-800 nm), air cooled at ∼46 °C, with an intensity of 95 (mW/cm2) measured with a power meter from the LSI Corp., Yellow Springs, CO. Nafion strips of 5 cm2 were placed immediately behind the wall of the reaction vessel. In experiments with 2-propanol, two Philips 36 W luminescent lamps centered at 366 nm, with a light flux of 2 mW/cm2, were used as the light source. UV-visible absorption spectra and total organic carbon were measured during the photocatalytic degradation. Analysis of Irradiated Solutions and Catalyst Characterization Techniques. The absorption spectra were recorded using a Hewlett-Packard 8452 diode-array spectrophotometer. The total organic carbon (TOC) was monitored via a Shimadzu instrument equipped with an ASI automatic sample injector. The peroxide concentrations were assessed with Merkoquant paper at levels between 0.5 and 25 mg/L of H2O2. Infrared spectra were obtained in the attenuated total reflection infrared (ATRIR) mode on a Nicolet Magna DSP 650 instrument equipped with the Golden Gate accessory. Correction of the absorbance changes due to a variation in penetration depth (14) Cherepy, N. J.; Liston, D. B.; Lovejoy, J. A.; Deng, H. M.; Zhang, J. Z. J. Phys. Chem B 1998, 102, 770.
Langmuir, Vol. 18, No. 23, 2002 9055 with wavelength in the ATRIR mode were carried out with the adequate software. X-ray Photoelectron Spectroscopy (XPS) and Mo1 ssbauer Spectroscopy. XPS was carried out using a Leybold-Heraeus instrument referenced to the Mg KR1,2 line at 1253.6 eV. The binding energies of the iron oxide surface species were referenced to the Au 4f7/2 level of 83.8 eV. The quantitative evaluation of the experimental data was carried out with the well-known Shirley type correction of the background. This correction was necessary due to the strong electrostatic charging of the particles during the measurements. Mo¨ssbauer spectroscopy absorption was measured using a conventional constant acceleration spectrometer from Wissel (Germany) provided with (a) a source of 57Co in Rh and (b) a CF-506 cryostat from Oxford Instruments. The spectra were recorded at 80 K for all samples. Isomer shifts at this temperature were quoted relative to an absorber of metallic iron, typically in the range of 0.4-0.6 mm/s for Fe3+. The Mo¨ssbauer spectra peaks were computed using a curve fitting program provided with a Lorentzian peak generator since only uniform widening of the peaks was involved when deconvoluting the Mo¨ssbauer peak signals. Femtosecond Spectroscopy on Fe3+-Nafion Membranes and r -Fe2O3 Nanocrystalline Colloids. Experiments were performed by using a femtosecond pump-probe. Details of the femtosecond technique used have been described recently.15 The femtosecond mode locked (CPM) pulsed dye laser oscillator generates pulses with an energy of 1-2 nJ per pulse. The pulse duration is ∼70 fs full width at half-maximum (fwhm). The wavelength of the pulses was centered at 616 nm. The femtosecond pulses passed through a two-stage pulsed dye amplifier reaching 300-400 µJ per pulse at 25 Hz. The second harmonic centered at 308 nm with energy of 0.5 µJ was used for sample excitation. The optical length of the sample was 1 mm. The recorded transient spectra were time-corrected as previously reported.16 Experimental conditions for sample concentrations and the laser pulse energy were adjusted to minimize the background transient signal arising from the Nafion membrane. In this way, it was possible to avoid the effect on the iron oxide transient spectra due to the Nafion transient absorption. MatLab 5.1 and Igor Pro 4 were used to carry out the numerical analysis of the experimental data. The experiments with R-Fe2O3 were performed at pH ) 4. Transmission Electron Microscopy (TEM) and X-ray Energy Dispersive Spectrometry. Thin TEM foils were prepared by cryo-ultramicrotomy. Small pieces of the sample were embedded in epoxy and cut at liquid nitrogen temperature with a diamond knife (Diatome, 45° cutting edge) and a Reichert Ultracut E/FC4D ultramicrotome. Despite the fairly low temperature, the remaining plasticity of the Nafion did not allow to obtain sections thinner than 100 nm. TEM observation was performed on a Philips CM300UT/FEG microscope fitted with a Schottky field emission gun operated at 300 kV and with an objective lens with a very short spherical aberration coefficient (0.65 nm). The nominal resolving power at Scherzer defocus and the limit of information for very thin samples (“phase samples”) are 0.17 and 0.11 nm, respectively. The images were recorded on a Gatan 797 slow scan CCD camera with a numeric resolution of 1024 pixels × 1024 pixels with 14 bits modulation depth and handled with the Digital Micrograph 3.6 software (Gatan). Exposition of the sample to the electron beam was found to induce morphology changes. First, at low irradiation dose, the irradiated area becomes more transparent in bright field contrast. This was already noticed during the very early sample illumination stages with an intensity that was quite low for visual inspection at a magnification of 30 000 times. This can be taken as due to the degradation of the polymer matrix. Second, a coarsening of the catalyst particles occurs after a longer exposure or under a higher beam intensity probably due to particle sintering and recrystallization. Thus great care was taken to record images (15) Gostev, F. E.; Kachanov, A. A.; Kovalenko, S. A.; Lozovoi, V. V.; Panov, S. I.; Sarkisov, O. M.; Sviridenkov, E. A.; Titov, A. A.; Tovbin, D. G. Instrum. Exp. Tech. 1996, 39, 567. (16) Antipin, S. A.; Petrukhin, A. N.; Gostev, F. E.; Marevtsev, V. S.; Titov, A. A.; Barachevsky, V. A.; Strokach, Y. P.; Sarkisov, O. M. Chem. Phys. Lett. 2000, 331, 378.
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from areas exposed as little as possible. During image focusing, the illumination beam was restricted to a field corresponding to twice the CCD camera detector size to prevent damage in adjacent areas, keeping the beam intensity as low as possible. Subsequently, a fresh adjacent field was brought within the observation field by means of an electronic image shift, and at the same time the record knob of the CCD camera was activated and the picture was taken with an exposure time of 0.5 or 1 s. Under such conditions, the increase of the sample transparency was minimized for the postobservation of radiation damage. Besides this irreversible modification of the sample, irradiation induces also a flickering of the particle contrast that corresponds to sudden changes of its crystallographic orientation with respect to the electron beam. This effect has been reported in several cases for particles weakly bound to their matrix, as in the case for gold nanoparticles on amorphous carbon films. Modeling of the momentum transfer between the electron beam and the supported particle has been reported.17 Semiquantitative chemical analysis was performed by EDS energy-dispersive X-ray spectrometry (Oxford Inc. U.K.) with an organic detector window. Probes having 10 µm diameter were tested on weakly irradiated samples to monitor the sample composition. Analysis of individual particles was also attempted with probes of 2-8 nm in diameter, but without success. Partial destruction of the sample was observed due to holes in the matrix before being able to collect enough data to evaluate accurately the sample. Thus, electron diffraction became mandatory for phase identification of the catalyst particles. Conventional selected area electron diffraction (SAED) was then considered as a convenient tool for the analysis of the samples. Adverse effects during these measurements were observed due to (a) sample thickness, (b) dispersion of the particles in the amorphous polymer matrix, and (c) broadening of the diffraction reflections by the small particle size. The ring patterns superimposed on an intense background showed a contrast that was sufficient for accurate phase determination. Nanodiffraction was tried, but the high electron beam density brought unacceptable irradiation damage as for EDS. Eventually, crystallographic information was extracted from the observed diffraction contrast in HRTEM images. However, the amorphous matrix led to a grainy background compared to the “atomic plane image”. Thus, the measurement of interplanar spacings was carried out by way of a power spectrum obtained by fast Fourier transform (FFT) of the crystal image. The power spectra obtained resembled the diffraction patterns. The microscope resolving power used was sufficient to transmit the information obtained at high spatial frequencies. The accuracy of the spacing measurement is mainly restricted by the sampling of the image (1024 × 1024 pixels) and by the size of the camera pixels taken back to the sample level (0.021 nm/pixel at 780 000 times magnification, that is, about 5-10 pixels per interplanar spacing). To get statistically relevant data, several hundred spacings were measured for both samples and compared to the spacings present in the expected phases. From the EDS results, Fe, F, O, and H were considered as elements in the particles under study. The PDF file database (Int. Center Diffraction Data, release 2000) searched all the phases containing two or more of these elements and those containing at least one of the three largest interplanar spacings between 0.33 and 0.36 nm indicated by the raw HRTEM data. This analysis led to 20 structure files. The same search was also performed in the ICSD (Inorganic Crystal Structure Database, FIZ Karlsruhe and Gmelin-Institute, 1997) database and with random search of the spacings abd brought another 52 files. After removing the files describing very close structures, a database of 47 possible phases was attained for comparison with the experimental data. When comparing the experimental interplanar spacings found with the calculated values for a defined set of phases, statistical uncertainties greatly complicate this task. In addition, uncertainty in the microscope magnification and the diffraction camera length calibrations due to hysteresis in the magnetic lenses and actual objective lens current have to be taken into account when evaluating the sample height position in the goniometer. The experimental data are shifted but not in the same direction. As (17) Marks, L. D.; Zhang, P. J. Ultramicroscopy 1992, 419.
Kiwi et al. long as magnification and sample position are not changed, this shift remains constant and appears as a systematic error for the whole set of images. The numeric analysis is further complicated when a high number of phases have to be considered and mixed. A faster and interactive method was developed to compare graphically the reciprocal of the experimental reflection spacings of the most probable crystalline phases present in the samples. Matching of the phases was carried out by visual inspection, and the phases that did not follow the experimentally observed ones were neglected. Errors in calibration were corrected by rescaling the experimental data, and the interplanar spacings were obtained in this way from HRTEM analysis. A Gaussian shaped “diffraction reflection” with a width corresponding to the crystal diameter broadening was attached to each experimental spacing, and all the individual reflections were summed. The positions of the maxima of this sum correspond to the spacings and are thus equivalent to the position of the diffraction reflections observed on the intensity profile across a SAED pattern. It should be noted in this analysis that (i) the “intensities” are a measure of the occurrence during the observation of spacings, (ii) the limit of resolution of the microscope introduces a cutoff toward the small spacing, and finally (iii) some reflections may be missing if the microscope transfer functions. Finally, HRTEM images of crystals seen along a highsymmetry axis (“zone axis”) bring additional information not yet considered as the angle between two or more plane families. This was used to crosscheck the results of “pseudo-diffraction” analysis by performing full zone-axis indexation on their FFT powder spectra with the JEMS software by P. Stadelmann (http:// cimewww.epfl.ch/people/Stadelmann/jemsWebSite/jems.html). Estimation of Average Number of SO3- Ions in the Cluster. Equation 1 for Hmax gives an estimate of the number of headgroups in a single cluster by considering the average parameters such as density, Ew, Bragg d spacing, and adsorbed volume. The number of headgroups in a single cluster Hmax is calculated by
Hmax )
N0F v Ew 1 + V
(1)
where F ) 1.98 g/cm3 is the Nafion density, Ew ) 1100 is the equivalent weight, and v ) 4πR3/3 (R ) d/2) refers to the SO3-water cluster volume.18 The Bragg d spacing used was 47 Å, and V, the water volume absorbed by 1 cm3 of Nafion, was equivalent to 0.56 cm3. Taking into account the preceding data and Hmax ) 73, a maximum of 73/n Mn+ ions could be loaded into a single cluster. The concentration of clusters c (in units of mol/cm3) is calculated according to c ) F/EwHmax giving a value of about 2.5 × 10-5 mol/cm3. The average number of ions in a cluster (the occupation number) 〈F〉 ) ([ions]/c × 1000). The concentration of ions noted as [ions] refers to the species in the Nafion membrane.
Results and Discussion Photocatalytic Activity of Different Fe3+-Nafion Membranes. Features of Fe3+-Nafion Fenton-like catalytic systems are summarized in Figures 1-3: (1) Figure 1 shows a higher photocatalytic activity for the yellow membrane relative to the brown one for 2-propanol photodegradation and the total lack of 2-propanol degradation in the absence of light irradiation (trace 3 of Figure 1). (2) Figures 2 and 3 show under different experimental conditions the decoloration of Orange II mediated by the Fe3+-Nafion/H2O2/light system. Results from Figures 2 and 3 reveal a higher activity of the yellow Fe3+-Nafion relative to the brown one during the Orange II decoloration and mineralization. Despite the fact that some difference in activity exists in brown membranes when NaOH or ammonia water was used as a base during their preparation as shown by Figures 2A,B or 3, yellow membranes showed significantly higher photocatalytic activity relative to the brown ones. Therefore, we will (18) Lee, P. S.; Meisel, J. J. Am. Chem. Soc. 1980, 102, 5477.
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Figure 1. Kinetics of the TOC degradation for 2-propanol in the presence of H2O2. Trace 1: yellow Fe3+-Nafion membrane under 366 nm light irradiation; Fe3+ ) 1.3%. Trace 2: brown Fe3+-Nafion membrane under 366 nm light; Fe3+ ) 1.3%. NaOH was used during the pretreatment. Trace 3: yellow Fe3+-Nafion membrane in the dark; Fe3+ ) 1.3%. Other experimental conditions: pH ) 4 and [H2O2] ) 0.01 M.
Figure 3. (A) Kinetics of Orange II decoloration at λ ) 486 nm. Other experimental conditions: pH ) 3.5, [H2O2] ) 0.01 M, Fe3+ ) 1.5%. Trace 1: yellow Fe3+-Nafion. Trace 2: brown Fe3+-Nafion. Ammonia in water was used for the pretreatment. Trace 3: brown Fe3+-Nafion. NaOH was used for the pretreatment. (B) Kinetics of the mineralization of Orange II. Other experimental conditions were as in (A).
Figure 2. Orange II decoloration under Suntest irradiation. pH ) 3.5; [H2O2] ) 0.01 M; Fe3+ ) 1.5%. (A) Brown Fe3+Nafion. Ammonia was used for the pretreatment. (B) Brown Fe3+-Nafion. NaOH was used for the pretreatment. (C) Yellow Fe3+-Nafion.
address the detailed study of the Fe3+ variety existing in the yellow and brown Fe3+-Nafion membranes during the present study.
The exponential decrease of the TOC in Figure 1 allows us to compare the rates of 2-propanol oxidation by H2O2 in the presence of Fe3+-Nafion membranes loaded with different Fe3+ ion amounts. An optimum for the catalytic activity is reported in Figure 4 and is seen to be related to the Fe3+ content of the membrane. This is observed both for yellow and for brown Nafion forms; however, the activity of the yellow membrane is seen to be higher than that of the brown membrane. The identification of the iron clusters formed in the brown Nafion membrane will be carried out with reference to colloidal iron oxide with known structural features commonly available from common sources. This will be used to elucidate the structure-activity correlation for the Fe-Nafion membranes focused on during this study. Ion Exchange between H+ and Fe3+ in Yellow Nafion Membranes. Before carrying out a detailed spectroscopic analysis of Fe3+-Nafion, the amount of iron ions loaded by ion exchange on the Nafion was estimated. Taking into account the requirement of electroneutrality in the Nafion structure, it should be possible to suggest iron complexes in Fe3+-Nafion from the limiting loading of Fe3+ ions. The amount of Fe3+ adsorbed by Nafion was determined from the Fe3+ found in solution before and
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Figure 4. Dependency of the reciprocal lifetime of the TOC decay k(TOC) as a function of the partial weight of Fe3+ in the Nafion membrane during 2-propanol degradation. Other experimental conditions: pH ) 4.0, [H2O2] ) 0.01 M. 2-Propanol TOC initially was 14 ppm; 366 nm light. Trace 1: yellow Fe3+Nafion membrane. Trace 2: brown Fe3+-Nafion membrane.
Figure 5. Reciprocal dependence of the Fe3+ adsorbed by H+Nafion on the Fe3+ concentration in aqueous solution. Other experimental conditions: pH ) 1.8, 5 cm2 Nafion in 25 mL of aqueous solution.
after equilibration with Nafion. Figure 5 shows the concentration of iron ions in equilibrium with the Nafion membrane versus the amount of the Fe3+ adsorbed by Nafion. The limit value of the Fe3+ adsorbed by Nafion at infinitesimal large Fe3+ concentration was determined as the intercept for 1/Fe3+ads versus 1/[Fe3+] and found to be 7.6 ( 0.8 µM of Fe3+/cm2 Nafion or 1.8 ( 0.2% of the weight of the Nafion. The average number of SO3- groups per cluster Hmax was found by eq 1 to be 73, and the maximum amount of ions that could be loaded into a single cluster was found to be 73/n Mn+. Possible Mn+ ion monomers such as Fe(H2O)63+ or [FeOH(H2O)5]2+ and dimers [Fe-O-Fe]4+ and [Fe(H3O2)Fe]5+ are present in aqueous bulk solution under the experimental conditions used. The number of SO3- groups found per 1 cm2 of Nafion was Q ) V[cluster]Hmax, where the cluster concentration [cluster] ) 2.5 × 10-5 M/cm3. The volume V of 1 cm2 of Nafion 117 is equal to V ) 0.0117 cm3. According to this, Fe3+ can enter the membrane (a) in the form of [Fe(H2O)6]3+ up to Q × 1/3 ) 7.1 µM/cm2, (b) in the form of [FeOH(H2O)5]2+ up to Q × 1/2) 10.65 µM/cm2, or (c) in the form of a binuclear complex [Fe-O-Fe]4+ with Q × 2/4 ) 10.65 µM/cm2. If iron exists in the membrane in the form of binuclear hydroxo-complexes [Fe(H3O2)Fe]5+, then this value should be Q × 2/5 ) 8.52 µM/cm2. The experimental value obtained for the maximum iron loading is 7.6 µM/
Figure 6. (A) UV-vis absorption spectra of the water-swollen Fe3+-Nafion membranes at different conditions. Trace 1: brown Fe3+-Nafion membrane. Fe3+ loading ) 1.5%. NaOH was used for the pretreatment. Trace 2: brown Fe3+-Nafion membrane. Fe3+ loading ) 1.5%. Ammonia was used for the pretreatment. Trace 3: yellow Fe3+-Nafion membrane. Fe3+ loading ) 0.5%. Trace 4: yellow Fe3+-Nafion membrane. Fe3+ loading ) 1.6%. (B) Second derivative UV-vis absorption spectra of the Fe3+Nafion membranes under experimental different conditions. Trace 1: brown Fe3+-Nafion membrane. Fe3 loading ) 1.5%. NaOH was used for the pretreatment. Trace 2: brown Fe3+Nafion membrane. Fe3+ loading ) 1.5%. Ammonia was used for the pretreatment. Trace 3: yellow Fe3+-Nafion membrane. Fe3+ loading ) 0.5%. Trace 4: yellow Fe3+-Nafion membrane. Fe3+ loading ) 1.6%.
cm2. This value is closer to the values 7.1 µM/cm2 [Fe(H2O)6]3+ or 8.5 µM/cm2 [Fe(H3O2)Fe]5+ than to 10.7 µM/cm2 [FeOH(H2O)5]2+. Therefore, it is possible to suggest that the yellow Nafion membranes are mainly loaded with [Fe(H2O)6]3+ and [Fe(H3O2)Fe]5+ complexes. Spectroscopic observations shown in the next section below will confirm this suggestion. UV-Vis Spectra of Fe3+-Nafion Membranes. Figure 6 shows the spectra of Fe3+-Nafion water-swollen membranes prepared in three different ways: (1) Yellow Membranes. At a low level loading of Fe3+ in Nafion (less than 30% of the saturation), a prominent peak at 334 nm is observed, that can be attributed to the formation of the binuclear Fe3+ complex in Nafion (see trace 3 in Figure 6A). The peak at 334 nm does not depend on the counterion Cl- or ClO4- of the Fe ion in the solution. It matches the spectrum reported for the binuclear Fe3+
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Table 1. Peak Positions for Different Iron Oxides UV-vis hematite R-Fe2O3 geothite R-FeOOH lepidrocite γ-FeOOH akaganite β-FeOOH maghemite γ-Fe2O3 ferrihydrite Fe5HO8‚4H2O Fe polymer
peak, cm-1
peak, cm-1
IR peak, cm-1
18 450 (23) 20 330 (23) 20 450 (23) 19 920 (23) 20 200 (23) 19 530 19 840 (23)
18 900 (39) 20 200 (39) 20 620 (39)
625 (27) 896, 797 (27) 1010 (27) 1300 (27) 670 (27)
19 570 (23)
19 100 (39)
530 (27)
complex in aqueous solution.19 This spectrum differs from spectra of mononuclear Fe3+ complexes: [Fe(H2O)6]3+ at 240 nm, [Fe(H2O)5OH]2+ at 297 nm, and [Fe(H2O)4OH2]+ at 297 nm.20,21 Moreover, growth kinetics at λ ) 334 nm was observed when the membrane was immersed in the Fe3+ solution for a relatively short time (30 min) and equilibrated afterward with neutral distilled water due to the binuclear Fe3+ complex formation. The formation of the binuclear Fe complex as shown in eq 2 is confirmed by Mo¨ssbauer spectroscopy below. The band at 334 nm can be attributed to the binuclear [Fe(H3O2)Fe]5+ complex
2[Fe(H2O)6]3+ + H2O f [(H2O)5Fe(H3O2)Fe(H2O)5]5+ + H+ or
2[Fe(H2O)5OH]2+ + H+ f [(H2O)5Fe(H3O2)Fe(H2O)5]5+ (2) (2) Yellow Membrane. If the Fe3+ concentration on the Nafion membrane is above 30% and up to the saturation limit, an increase of monomers and dimers of Fe3+ in Nafion results in the shoulder band shown in Figure 6A at λ > 400 nm (trace 4), which can be assigned to Fe3+ oxide oligomers or nanocrystallites. This band is responsible for the yellow color of the membrane. A more detailed analysis of this shoulder will be presented below. (3) Brown Membranes. Figure 6A also presents the absorption spectra of the Fe3+-Nafion membrane after immersion in NaOH (trace 1) or ammonia (trace 2) leading to a deep brown color due to precipitation of the Fe3+ ions in the membrane. The Fe3+-Nafion membrane prepared by Fe3+ exchange with the H+-Nafion or the Na+-Nafion led to spectra that are very similar in shape. These spectra are structured and are seen to be different for Fe oxide crystals and Fe polymers (oligomers). To improve the spectral resolution and the identification of the different Fe oxides, second derivatives were used for the spectra analysis. The 2(6A1) f 2(4T1(4G)) transition has been reported at different positions in various study of Fe oxides; it can be used as the reference absorption band for the determination of the type of Fe oxide involved.22,23 In Figure 6B, the positions of the 2(6A1) f 2(4T1(4G)) maxima are at 19 960 cm-1 (554 nm) and 17 794 cm-1 (562 nm) for the wide absorption tail of the yellow form of the Fe3+Nafion membranes swollen by water. The band at 554 nm is close to the band reported for the Fe oxide polymer and (19) Knight, R. J.; Sylva, R. N. J. Inorg. Nucl. Chem. 1975, 37, 779. (20) Byrne, R. H.; Kester, D. R. J. Solution Chem. 1978, 7, 373. (21) Byrne, R. H.; Kester, D. R. J. Solution Chem. 1981, 10, 51. (22) Cornell, R. M.; Schwertmann, U. The Iron Oxide. Structure, Properties, Reactions, Occurrence and Uses; VCH: Weinheim, 1996; pp 1-573. (23) Malengreau, N.; Muller, J. P.; Calas, G. Clays Clay Miner. 1994, 42, 137.
Figure 7. FTIR spectra of dry and water-swollen Fe3+-Nafion membranes for the yellow and brown forms in the region 5004000 cm-1. The FTIR spectrum of the H+-Nafion was used used as a reference. (A, dry samples) Trace 1: yellow Fe3+Nafion; Fe3+ ) 1.7%. Trace 2: H+-Nafion. Trace 3: brown Fe3+-Nafion; Fe3+ ) 1.7%. NaOH was used. (B, water-swollen samples) Trace 1: yellow Fe3+-Nafion; Fe3+ ) 1.7%. Trace 2: H+-Nafion. Trace 3: brown Fe3+-Nafion; Fe3+ ) 1.7%. NaOH pretreated.
oligomer forms as shown in Table 1 and references noted therein. The spectrum of the brown form of the Fe3+Nafion has peaks at 20 576 cm-1 (486 nm) and 18 050 cm-1 (554 nm). These two bands are close to the bands of lepidrocite (γ-FeOOH) or hematite (R-Fe2O3) (see Table 1). FTIR Spectra of Fe3+-Nafion Membranes. Additional information about iron complexes and oxide forms in the Fe3+-Nafion can be obtained from FTIR spectra for dry and water-swollen Fe3+-Nafion membranes. Figure 7 A,B shows the FTIR wide-range spectral data for the yellow and brown forms of the Fe3+-Nafion and H+Nafion. These spectral observations of H+-Nafion are consistent with observations on H+-Nafion membranes recently reported.23-26 The region of 1400-1100 cm-1 corresponds to strong C-F symmetric and antisymmetric vibrations. The vibrations at 630 and 530 cm-1 are (24) Laporta, M.; Pegoraro, M.; Zanderighi, L. Phys. Chem. Chem. Phys. 1999, 1, 4619. (25) Ludvigsson, M.; Lindgren, J.; Tegenfeldt, J. Electrochim. Acta 2000, 45, 2267. (26) Xie, G.; Okada, T. Zeit. Phys. Chem.sInt. J. Res. Phys. Chem. Chem. Phys. 1998, 205, 113.
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Figure 9. Mo¨ssbauer spectra of the Fe3+-Nafion samples. (1) Brown membrane. Sample 1 was prepared by the ion exchange between H+ and Fe3+ with the subsequent pretreatment by NaOH. (2) Yellow membrane. The Fe3+-Nafion was prepared as an exchange between Na+ and Fe3+ by the immersion of Na+-Nafion in the Fe3+ solution. Na+-Nafion was prepared by the immersion of the H+-Nafion membrane in NaOH. (3) Yellow membrane. Sample 3 was prepared by the ion exchange between H+ and Fe3+ in the Nafion membrane.
Figure 8. FTIR spectra of dry Fe3+-Nafion membranes for the yellow and brown forms in the region 500-1100 cm-1. The FTIR spectrum of the H+-Nafion is used as a reference. (A, water-swollen samples) Trace 1: yellow Fe3+-Nafion; Fe3+ ) 1.7%. Trace 2: H+-Nafion. Trace 3: brown Fe3+-Nafion; Fe3+ ) 1.7%. NaOH was used. (B, dry samples) Trace 1: yellow Fe3+-Nafion; Fe3+ ) 1.7%. Trace 2: H+-Nafion. Trace 3: brown Fe3+-Nafion; Fe3+ ) 1.7%. NaOH used for pretreatment. (C) Differences between spectra of Fe3+-Nafion and H+-Nafion membranes. Trace 1: yellow form. Trace 2: brown form.
assigned to C-F stretching vibrations. A band at 1060 cm-1 corresponds to the symmetric stretching vibrations of the SO3- group. The wide band at 3400 cm-1 is attributed to the free water. FTIR bands of the iron oxides in Fe3+Nafion are weak. They can be seen in Figure 8 in the 870 and 625 cm-1 peaks. The band at 625 cm-1 indicates the presence of some hematite and lepidrocite in the yellow Fe3+-Nafion22,27 membrane. The intensity of the 870 cm-1 band is higher in the dry membrane than in the case of the water-swollen membrane. This band has been assigned previously to oxo-bonded Fe-O-Fe.28 This latter band increases its intensity due to dehydration and can be attributed to the iron [Fe-O-Fe]4+ dimer in the yellow Fe3+-Nafion.. Iron-57 Mo1 ssbauer Studies of Fe3+-Nafion. Figure 9 presents Mo¨ssbauer spectra for the yellow and brown samples. The three main spectral components of the yellow membrane are the two doublets D1 and D2 and the doublet D3 and the sextuplet M1 for the brown form of the Fe3+(27) Jasinski, R.; Iob, A. J. Electrochem. Soc. 1987, 134, C424. (28) HeitnerWirguin, C. Polymer 1979, 20, 371.
Nafion membrane. These results are presented in Table 2. The obtained spectra agree with the ones reported in the literature.29-36 There is agreement in the literature on the assignment of the doublet D2 (δ ) 0.55 mm/s, ∆ ) 1.66 mm/s) to [Fe-O-Fe]4+ oxo-bridged dimers.29-36 At the same time, the doublet D1 (δ ) 0.49 mm/s and ∆ ) 0.44 mm/s) was identified as having in its structure the [Fe(H3O2)Fe]5+ dimer due to aqua Fe3+ ions bridged by a H3O2- (hydrogen oxide) ligand.34,35 But a variance in the assignment of the M1 components of the Mo¨ssbauer spectra for the particular Fe3+ species formed in the Nafion membrane has been reported. The sextuplet M1 was attributed to the Fe3+-aqua complex consisting of iron clusters of variable size containing amorphous iron.30 As described above, the intensity of M1 increased after HOwas added to the solution. This strongly suggests the assignment of M1 to a precipitated iron species with iron oxide particles of 10-50 Å with an observed magnetic hyperfine splitting of H ) 486 kG. This is less than H ) 560 kG, the value for the Fe3+ hexa-aqua complex, and differs also from the value for bulk hematite of H ) 541 kG at 85 K. A comparison of UV-visible, FTIR, and Mo¨ssbauer spectra allows one to identify and separate out some binuclear Fe3+ oxide complexes in the SO3--water clusters: (a) [Fe(OH)2Fe]4+; (b) [Fe-O-Fe]4+; (c) [Fe(H3O2)Fe]5+. Fe3+ dimers in solution as iron-hydroxo [Fe(OH)2Fe]4+ species have been suggested as being responsible for the UV-vis band absorption at 334 nm,19 as well as the iron oxo-bridged [Fe-O-Fe]4+ species. (29) Chibirova, F. K.; Zakharin, D. S.; Sedov, V. E.; Timashev, S. F.; Popkov, Y. M.; Reiman, S. I. Khim. Fiz. 1987, 6, 1137. (30) HeitnerWirguin, C. Polymer 1980, 21, 1327. (31) Meagher, A.; Rodmacq, B.; Coey, J. M. D.; Pineri, M. React. Polym. 1984, 2, 51. (32) Meagher, A.; Rodmacq, B. New J. Chem. 1988, 12, 961. (33) Meagher, A. Inorg. Chim. Acta 1988, 146, 19. (34) Pan, H. K.; Yarusso, D. J.; Knapp, G. S.; Pineri, M.; Meagher, A.; Coey, J. M. D.; Cooper, S. L. J. Chem. Phys. 1983, 79, 4736. (35) Pan, H. K.; Meagher, A.; Pineri, M.; Knapp, G. S.; Cooper, S. L. J. Chem. Phys 1985, 82, 1529. (36) Rodmacq, B.; Pineri, M.; Coey, J. M. D.; Meagher, A. J. Polym. Sci., Polym. Phys. Ed. 1982, 20, 603.
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Table 2. Mo1 ssbauer Spectral Components of the Fe in Yellow and Brown Membranes D1 δ (mm/s) 1 2 3
0.49 0.49
∆ (mm/s) 0.43 0.44
D2 % 46.0 42.7
δ (mm/s) 0.55 0.55
∆ (mm/s) 1.66 1.66
Knudsen et al.37 indicated that the D2 binuclear iron(III) complex is an oxo rather than a dihydroxo complex. The oxo complexes show a quadrupole splitting of 1.7 mm/s, whereas this value for the dihydroxo species is 0.8 mm/s. The observation by FTIR in the present work that the Fe-O-Fe band increases due to the dehydration of the Nafion is in agreement with [Fe(H3O2)Fe]5+ changing into [Fe-O-Fe]4+-aq as the Nafion is dried.32 Therefore, the dimer found in the swollen Fe3+-Nafion should be the same in the solution showing a peak at 334 nm. This peak should be attributed to [Fe(H3O2)Fe]5+ or [Fe-O-Fe4+] rather than to the species [Fe(HO)2Fe]4+ as previously reported.24 Extended X-ray absorption fine structure (EXAFS) work on Fe3+-Nafion membranes reports Fe3+ dimers in Nafion as being the precursors of ferric hydroxide and ferric oxide precipitates due to the addition of HOin solution.35 This is in agreement with the results obtained during this study where the rise of the intensity of the sextuplet M1 was observed when Fe3+-Nafion underwent a change from the yellow Nafion to brown Nafion in basic media. XPS of Fe-Nafion Membranes of the Yellow and Brown Varieties before and after Orange II Photodegradation. The oxidation state determination of the exchanged iron during the reaction was carried out by XPS. Corrections for electrostatic charging of the particles during the measurements were carried out by internal referencing to the aliphatic surface carbon at 284.6 eV and by cross-checking the peak distances between C 1s, O 1s, and Fe 2p3/2. Gaussian-Lorentzian fitting of the XPS peaks was carried out by the Shirley type background correction subtracting the X-ray satellite peaks. Polynomial second-order fits of the experimental curves were carried out to match the asymmetric shape of the corrected XPS signals and referenced to the Fe 2p3/2 binding energies (BEs): Fe metal, 706.7 eV; FeO, 709.6 eV, Fe2O3, 710.9 eV; Fe3O4, 711.4 eV. No evidence for the zero oxidation state of Fe was found. The BEs for the Fe 2p3/2 did vary by C• Fe3+ + F- f FeF2+ + ...
(3)
Apparently the lower concentration of FeF2 phase in the brown relative to the yellow membranes is explained by that fact that in the brown membrane Fe is bound in nanocrystallites of Fe2O3 whereas in the yellow membrane it is in the form of monomer and dimer complexes. Formation of FeF2 from the monomers and dimers would be preferred compared to nanocrystallites of Fe2O3. Femtosecond Laser Spectroscopy of Iron Oxides in Fe3+-Nafion and in r-Fe2O3 Colloidal Nanocrystallites. The yield of the long-lived excited states in the nanosecond, microsecond, or longer time scale is low for the excited Fe3+-Nafion yellow and brown samples. A transient absorption signal was not detected in the experiments with the nanosecond laser photolysis excitation (λexc ) 347 nm). The same Fe3+-Nafion samples exhibited transients in the femto- and picosecond time scales. The dynamics of the relaxation was observed to be
Figure 11. Yellow Nafion sample: Comparison between the lattice spacings measured from the FFT power spectra of the yellow sample. Only FeF2 was observed to match the peaks (plain lines on negative side). Fe2O3 hematite is not observed in this sample. Some small peaks remain unexplained by all phases of the set. Particles in the yellow Nafion were 3-4 nm in size. Smaller particles cannot be excluded but could not be detected accurately.
different for yellow and brown Fe3+-Nafion membranes. The femtosecond transient absorption profiles observed in brown Nafion membranes are presented in Figure 12, column A. For a comparison, column B of Figure 12 present the transients for aqueous nanocrystallite colloidal R-Fe2O3. The profiles of the transients observed in the yellow form of Fe3+-Nafion in column C of Figure 12 are seen to be different from the transient of the brown Fe3+-Nafion sample. At the same time, a similarity was observed between the profiles of the brown Fe3+-Nafion sample and colloid R-Fe2O3. In the brown Fe3+-Nafion sample and in the R-Fe2O3 colloid, a multiexponential decay was observed for the transient absorption, whereas in the yellow Fe3+-Nafion sample a prominent signal growth with a rise time close to 300 fs was observed. The latter growing in of the signal was significantly longer than the duration of the laser pulse. Afterward, multiexponential decay followed in the picosecond range. The time constant of the decay in the yellow Fe3+-Nafion sample was determined to be 30 ( 8 ps. Control experiments for the transient absorption were carried out with H+-Nafion membranes without Fe3+ additives. No meaningful signal was observed for the H+-Nafion membranes. This indicates that the observed transients are due to the Fe3+ transient absorption in the SO3- aqueous Nafion clusters. Transients shown in Figure 12 for the brown Fe3+-Nafion sample and colloidal R-Fe2O3 were fitted by a double exponential in the time domain of e3 ps: A1 exp(-t/τ1) + A2 exp(-t/τ2). The result of the fitting procedure for the brown Fe3+-Nafion membrane gave the values τ1 ) 320 ( 30 fs, τ2 ) 1.5 ( 0.9 ps, and A1/(A1 + A2) ) 0.68 ( 0.15. For the samples of R-Fe2O3, τ1 ) 360 ( 80 fs, τ2 ) 2.4 ( 2.0 ps, and A1/(A1 + A2) ) 0.60 ( 0.09. For both brown Fe3+-Nafion and R-Fe2O3 colloidal solutions, a longer decay component can be determined by using a threeexponential fit with a time constant close to 30 ps. The last decay has a lower amplitude, and it could not be determined accurately. The decrease in the transient absorption during 6 ps was observed to be close to 80%. The comparison between the time constants for transient decays of iron oxides in Fe3+-Nafion and aqueous colloidal R-Fe2O3 was in the experimental error limit. A lack of dependence of the transient profiles on the wavelength was observed for both the brown Fe3+-Nafion sample and the R-Fe2O3 colloid. The profiles of the transient decay for both the Fe3+-Nafion brown sample and the R-Fe2O3 colloid were found to be linear with the applied laser pulse energy in the range of 0.5-2.0 µJ. The lack of wavelength dependence for decay profiles for R-Fe2O3 in the range of
9064 Langmuir, Vol. 18, No. 23, 2002 Kiwi et al.
Figure 12. Transients observed in the brown Fe3+-Nafion membrane (column A); R-Fe2O3 nanocrystallite colloidal solution (column B); yellow Fe3+-Nafion membrane. Probe wavelengths are indicated. Excitation ) 70 fs, λ ) 308 nm laser pulse. Dashed lines show transient absorption profiles. Gaussian solid lines display the time function used in the laser probe measurements. Solid decay lines for panels in columns A and B through the experimental points show the best biexponential fit (A1 exp(-t/τ1) + A2 exp(-t/τ2)) to the experimental profiles. For column A: (1) probe wavelength λ ) 750 nm, τ1 ) 102 ( 20 fs, τ2 ) 4.4 ( 0.9 ps, A1/(A1 + A2) ) 0.71 ( 0.15; (2) probe wavelength λ ) 540 nm, τ1 ) 142 ( 30 fs, τ2 ) 4.5 ( 0.9 ps, A1/(A1 + A2) ) 0.65 ( 0.15; (3) probe wavelength λ ) 520 nm, τ1 ) 142 ( 30 fs, τ2 ) 4.5 ( 0.9 ps, A1/(A1 + A2) ) 0.65 ( 0.15. For column B: (1) probe wavelength λ ) 750 nm, τ1 )238 ( 80 fs, τ2 ) 5.3 ( 1.9 ps, A1/(A1 + A2) ) 0.65 ( 0.03; (2) probe wavelength λ ) 500 nm, τ1 ) 256 ( 70 fs, τ2 ) 9.1 ( 3.6 ps, A1/(A1 + A2) ) 0.58 ( 0.09; (3) probe wavelength λ ) 460 nm, τ1 ) 256 ( 70 fs, τ2 ) 9.1 ( 3.6 ps, A1/(A1 + A2) ) 0.58 ( 0.09. For column C: Solid rise-decay lines through the experimental points show the best biexponential approximation A1(1 - exp(-t/τ1)) exp(-t/τ2) to the experimental profiles.
Catalytic Fe3+ Clusters and Complexes in Nafion Langmuir, Vol. 18, No. 23, 2002 9065
Figure 13. Transient absorption spectra for the Fe3+-Nafion sample (column A) and for the R-Fe2O3 nanocrystallite colloid (column B). Time delays are indicated in the insets.
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660-850 nm after excitation with light at λ ) 390 nm (3.17 eV) has been reported before with time constants of τ1 ) 360 fs, τ2 ) 4.2 ps, and τ3 ) 67 ps.14 The time constants found for R-Fe2O3 with the excitation at λ ) 308 nm (4.03 eV) found during the present work were close to the values reported previously with laser excitation at λ ) 390 nm (3.17 eV). This indicates the lack of effect of the energy of the excitation photon on the decay time constants in the range of 3.17-4.03 eV. The transient spectra of the brown Fe3+-Nafion sample and the R-Fe2O3 colloid are shown in Figure 13. For both systems, wide bands are observed from 400 up to 900 nm. A similarity in the profiles of transient decay and in the transient absorption spectra between the brown Fe3+Nafion and the R-Fe2O3 colloid indicates that transient absorption in the brown Fe3+-Nafion membrane is determined by nanocrystallites of hematite already reported by various spectroscopic techniques and HRTEM. Moreover, it can be concluded that the R-Fe2O3 transient dynamics is not very much affected by aqueous surroundings or surroundings of SO3-- water clusters in Nafion or by a wide size distribution for the R-Fe2O3 colloid (225 nm) or a narrow distribution of R-Fe2O3 nanocrystallites in the Nafion due to the space restrictions of SO3--water clusters (3.5-5 nm). The observation of the fast relaxation during laser-induced experiments correlates with the low luminescent quantum yield observed for Fe3+-Nafion and R-Fe2O3 colloid samples. The qualitative difference between the transient dynamics for brown and yellow Fe3+Nafion membranes evidently corresponds to different phases present in SO3-- water clusters at almost the same amount of iron(III) in the clusters. In the brown membrane, iron(III) in the crystal phase R-Fe2O3 is present, but in the yellow membrane iron(III) is in the form of form of monomers and dimer aqua-complexes. Apparently, the transient absorption corresponds to both the ligand field transitions and the photogenerated charge carriers in the excited R-Fe2O3 nanocrystallites. It can be suggested that the fast relaxation with time constants close to 300 fs corresponds to nonradiative ligand field transitions due to the cross-relaxation mechanism.38 The fast time constant is explained by very high exchange interaction between adjacent Fe3+ ions through the O2ligand in the R-Fe2O3 crystal.39 The observed picosecond decay is in the range of the electron-hole recombination (38) Hu¨fner, S. Optical spectra of transparent rare-earth compounds; Academic Press: New York, 1978. (39) Scherman, D. M.; Waite, D. Am. Mineral. 1985, 70, 262.
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mediated by high-density ligand-field electronic states or traps.14 In any case, the excitation energy in R-Fe2O3 dissipates very fast. From photolysis of iron aqua complexes, it is known that ligand-metal charge transfer occurs due to photon absorption with the formation of iron(II) states and HO• radicals. Because the yellow membrane consists mainly of mono- and binuclear iron aqua complexes, the yellow Fe3+-Nafion membrane is a more suitable system to form the iron(II) and HO• radicals that are active intermediate species participating in the Fenton like catalysis. Conclusions The main forms of iron(III) in the Fe3+-Nafion can be summarized as monuclear complexes of [Fe(H2O)6]3+ and binuclear complexes [Fe(H3O2)Fe]5+ and [Fe-O-Fe]4+ in the yellow form of the Fe3+-Nafion membrane and the R-Fe2O3 nanocrystallite phase in the Fe3+-Nafion brown membrane. Spectral bands of the iron(III) forms were identified during this study by different techniques: UVvisible, FTIR, and Mo¨ssbauer spectroscopy. Electron microscopy (HRTEM) provided evidence for the formation of R-Fe2O3 nanocrystallites in the SO3--water clusters of Nafion. Some amount of oligomers of iron(III) species was detected by UV-visible spectroscopy at a high loading of Fe ions in the yellow membranes. The formation of low nuclear iron(III) complexes correlates with the decline in the activity of Fe3+-Nafion as can be seen in Figure 4. The formation of oligomer iron(III) correlates with a lower membrane photoactivity. The presence of R-Fe2O3 nanocrystallites in the SO3--water clusters of Fe3+-Nafion correlates with a significant decline of the activity of Fe3+Nafion. The dissipation of the photoexcitation occurs very fast in R-Fe2O3 nanocrystallites in the femtosecond and picosecond time domain and can explain the lower photoactivity of the R-Fe2O3 phase. As a conclusion, it could be suggested that iron(III) oligomers and R-Fe2O3 nanocrystallites are less active in the Fenton catalytic cycle FeIII + H2O2 f FeII + HO2• + H+, FeII + H2O2 f FeIII + HO• + HO- relative to the iron species [Fe(H2O)6]3+ and [Fe(H3O2)Fe]5+ or [Fe-O-Fe]4+. Acknowledgment. The authors acknowledge financial support by the KTI/CTI TOP NANO 21 (Bern, Switzerland) under Grant 5320.1 TNS and by the Russian Foundation for Basic Research under Grant No. 00-0332254 and No. 00-03-40118 and INTAS Grant No. 2000-554. LA020648K
