Article pubs.acs.org/JPCB
Radiation-Induced Chemical Changes to Iron Oxides Sarah C. Reiff and Jay A. LaVerne Radiation Laboratory and Department of Physics University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *
ABSTRACT: The radiolysis of a variety of iron oxide powders with different amounts of associated water has been performed using γ rays and 5 MeV 4He ions. Adsorbed water was characterized by both temperature-programmed desorption and diffuse reflection infrared Fourier transform spectroscopy to reveal a variety of active sites on the surface. Molecular hydrogen production was found only from water adsorbed on Fe2O3, and the yield was several orders of magnitude greater than that of bulk water. Aqueous slurries of FeO, Fe3O4, and Fe2O3 examined as a function of water fraction gave different yields of H2 depending on the oxide type and the amount of water. Examination of the iron oxide powders following irradiation by X-ray diffraction showed no change in crystal structure. Raman spectroscopy of the oxides revealed the formation of islands of Fe2O3 on the surfaces of FeO and Fe3O4. X-ray photoelectron spectroscopy of the oxides revealed the general formation of oxygen species following radiolysis.
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INTRODUCTION A number of recent studies have observed enhanced radiation decomposition of water when associated with various solid oxides.1−5 Iron oxides have a number of interesting biological and geological applications;6 a number of papers have examined radiation effects on stainless steel,7,8 carbon steel,9−12 magnetite,13 and pure iron12,14−16 under a variety of atmospheres, but radiation effects on the molecular level at iron oxide surfaces are not completely understood. Currently, stainless steel containers are being proposed for long-term radioactive waste storage.17 Over the lifetime of the container it will be exposed to a high level of radiation from decomposition of the waste inside. Of interest is how the structural integrity of the container is affected by the radiation, and if any byproducts are produced in the interaction of the radiation with the container material and any water that will be present. These topics fall under the general category of radiation-induced corrosion and represent a huge problem to understand from the fundamental aspects. The interaction between radiation and water is known to produce a number of reactive products including e−, H•, OH•, HO2•, H2O2, and H2.18 The production of H2 is of particular interest because it is one of the stable final products, and it is important in many practical applications for safety reasons. A number of previous experiments on the gamma radiolysis of metal oxides including, ZrO2,1,2,4,19 CeO2,2 and UO23 have shown that the production of H2 is increased in the presence of a metal oxide compared to the expected value from bulk water radiolysis. The exact mechanism for this increase in H2 yield is not known but probably involves energy transfer through the interface. Although many different solid oxides have been examined, there is still no clear method for predicting the water radiation chemistry when associated with these oxides.4 In this paper, the interaction of γ rays and 4He ions with three iron oxides, FeO, Fe3O4, and Fe2O3 has been investigated using a number of techniques. The oxide nanoparticles were characterized prior to irradiation using temperature-pro© XXXX American Chemical Society
grammed desorption (TPD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Samples were analyzed following irradiation to observe any changes in the chemical structure of the surface. The interaction of water, both attached water layers and bulk water, with the iron oxides was analyzed, and the production of H2 by radiolysis was measured.
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EXPERIMENTAL METHODS Sample Preparation and Surface Analysis. Iron(II) oxide (FeO, 99.5%), iron (II, III) oxide (Fe3O4, 99.997%), and iron(III) oxide (Fe2O3, 99.998%, alpha phase) powders were purchased from Alfa Aesar in the highest quality available. The Fe3O4 and Fe2O3 powders were fine enough to be used as received. FeO powders were crushed and filtered through a 170 mesh sieve to achieve a more uniform particle size. A Quantachrome Autosorb 1 was used to measure the particle size using the Brunauer−Emmett−Teller (BET) methodology. Surface porosity and particle size were visualized using a FEIMagellan 400 FESEM, field-emission scanning electron microscope. A voltage of up to 5.0 kV and magnification of 1200× were used to image the powders, which were mounted on SEM stubs and coated with 2 nm of iridium for conductivity. Two experimental methods, temperature-programmed desorption (TPD) and temperature-dependent diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), were used to analyze the water and other contaminants present on the oxide surfaces. For the TPD measurement, a custom cell was Special Issue: John R. Miller and Marshall D. Newton Festschrift Received: October 31, 2014 Revised: January 9, 2015
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DOI: 10.1021/jp510943j J. Phys. Chem. B XXXX, XXX, XXX−XXX
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Irradiations. Gamma ray irradiations were performed using a self-contained Shepard 60Co source located at the University of Notre Dame Radiation Laboratory, which had a dose rate in January 2014 of 179 Gy/min as determined using Fricke dosimetry. Sample cells consisted of 10 mm diameter standard NMR tubes by about 10 cm long that were flame-sealed following evacuation. Slurries were constantly rotated throughout the radiolysis to ensure uniformity. Heavy ion irradiations were performed with 5 MeV 4He ions produced in the Nuclear Science Laboratory at the University of Notre Dame. The beam diameter was 6.4 mm with a fluence of up to 1 × 1015 ions/cm2, as determined by integration of the beam on target and a total dose per sample of 137 MGy. Typical beam currents were 10 nA of charge, and no sample heating was detected. Sample cells were custom-made of Pyrex with a gas inlet and outlet and a mica window for beam entrance of typically 5 mg/cm2. Samples were irradiated in an atmosphere of ultrahigh-purity (99.9999%) argon.
used to hold between 50−100 mg of oxide powder in a crucible that was heated from room temperature to 500 °C at a rate of 2 °C/min. The initial pressure in the chamber was 10−10 bar before heating. The gases desorbing from the surface were measured using a Pfeiffer Prisma quadrupole mass spectrometer. Mass to charge ratios of 14, 15, 16, 18, 28, 32, 40, and 44 were monitored as a function of increasing temperature. A background measurement of an empty cell was measured prior to each sample and was subtracted from the sample measurements. A Bruker Vortex 70 instrument with a Harrick Praying Mantis high-temperature cell was used for the DRIFT measurements. This device is capable of obtaining in situ spectra of enclosed samples up to 500 °C. All oxides absorbed too much light to give reflectance spectra so they were mixed with KBr prior to analysis. The black FeO and Fe3O4 powders were diluted more than the red Fe2O3, and the relatively large amount of KBr may have affected the spectra. Raman spectroscopy measurements were taken using a Jasco Micro-Raman Spectrometer MRS-5100. In our configuration, the beam diameter on the sample is estimated to be around 1 μm with a 10 μm sampling depth. A 785 nm laser was used for analysis of the Fe2O3 sample, while a 532 nm laser was used for FeO and Fe3O4 samples. The higher power of the 785 nm laser was found to oxidize the FeO and Fe3O4 samples before a precise measurement could be made. Resolution for the Fe2O3 sample was 2.88 cm−1, and the laser power was 0.8 mW. For the FeO and Fe3O4 powders, the laser power was lowered to 0.3 mW to prevent oxidation20 and the resolution was 3.51 cm−1. Under these conditions, the spectra for FeO and Fe3O4 were not observed to change over the course of the measurement. X-ray photoelectron spectroscopy (XPS) was performed using a PHI VersaProbe II X-ray Photoelectron Spectrometer equipped with a monochromatic Al Kα X-ray source, with photon energy of 1486.6 eV, and a hemispherical electron energy analyzer. Wide scans over the binding energy range from 1486−0 eV were taken with a pass energy of 187.85 eV and an energy step of 0.4 eV. High-resolution scans were taken for each element of interest with a pass energy of 23.5 eV and energy steps of 0.05 eV. The sample spot analyzed was 100− 200 μm in diameter, and the sampling depth is estimated to be less than 10 nm. The sample chamber was also flooded with 10 eV Ar2+ ions and low-energy electrons to neutralize the sample during measurement. Under these conditions, the resolution for the wide range scans was 2.43 eV and was 0.58 eV for the elemental region scans as determined using the full width at half-maximum, fwhm, for the Ag 3d5/2 peak from a sputtered silver foil. X-ray diffraction (XRD) measurements were taken using a Bruker D8 Advance Davinci Powder X-ray Diffractometer. Cu Kα X-rays with an energy of 8.047 keV used. Scans were taken over a range of two-theta values from 20° to 80°, with a 0.05° step. Samples were also rotated at a rate of 15 rpm to prevent radiation damage during measurement. H2 Determination. The amount of H2 and other gases produced during irradiation were measured using a SRI 8610 gas chromatograph, GC, with the sample in an in-line mode and a thermal conductivity detector (TCD).2 The carrier gas used was ultrahigh-purity (99.9999%) argon, and the system had a sensitivity limit of 1 μL of H2. The error in gas measurement is estimated to be within 10%. The amount of H2 produced was found to be linear as a function of dose unless otherwise specified.
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RESULTS AND DISCUSSION Analysis of Surface Water and H2 Production by Radiolysis. Extensive characterization of the oxide surfaces was undertaken since water decomposition is thought to occur at or near to this interface.5,19 The specific surface areas for each oxide powder as determined using the BET method were 0.33 ± 0.18, 2.34 ± 0.04, and 2.62 ± 0.09 m2/g for FeO, Fe3O4, and Fe2O3, respectively. Multiple scans were used to determine the standard deviations. Average particle diameters could then be estimated as 4512, 437, and 486 nm by assuming perfect spheres for the FeO, Fe2O3, and Fe3O4 powders, respectively. A representative isotherm for each sample is shown in Figure 1. The shape of the isotherms as well as the lack of hysteresis between the adsorption/desorption curves suggest that the particles are either macroporous or nonporous.
Figure 1. Isotherms obtained from N2 adsorption (•) and desorption (■) for FeO, Fe3O4, and Fe2O3.
Visualization of the particle integrity was obtained by SEM analysis. SEM images of each oxide powder are shown in Figure 2. The FeO powders appear to be more spherically shaped than the Fe2O3 and Fe3O4 powders, which have a more porous, rod shape. In addition, the SEM images show agreement with the BET results in that the FeO particles have a greater particle diameter than the other oxides. The overall structure of the particles in the SEM image suggests that the oxide particles agglomerate together forming larger structures. All three powders appear to be smooth with no small pores. B
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Figure 4. Room temperature DRIFT measurements of FeO (second curve from top), Fe2O3 (third curve from top), Fe3O4 (lower curve). A reference spectra of H2O ice at 10 K is shown in the top curve.23 All oxides were mixed with KBr before analysis.
Figure 2. SEM images of FeO powders (top), Fe3O4 powders (middle), and Fe2O3 (bottom) at different magnifications.
during sample measurement. For the FeO and Fe3O4 samples, the DRIFT spectra are flat and do not show the characteristic peaks for OH bonding to the surface. The lack of observable OH species in the latter two powders is probably due to the interaction between the oxide and KBr used to dilute the sample for measurement, which was necessary due to the dark color and complete light absorption of the powders. The Fe2O3 sample was then heated to 400 °C, taking a DRIFT scan at various intervals, data not shown. Both the OH stretch and bend peaks were observed to decrease and then disappear as the temperature increased, suggesting that the water layers have desorbed from the sample by 400 °C. This observation is in agreement with the results from the TPD measurement which also showed water leaving the samples at temperatures less than 400 °C. Unfortunately, the broad OH stretch peak in the DRIFT spectra does not give any detailed information about specific binding sites. Two types of conditions were prepared for gamma irradiation and the measurement of the H2 produced. Oxide powders were dried and deposited in a 53% relative humidity chamber, maintained by a slurry of Mg(NO3)2 until the amount of water deposited on the samples was stable as determined by weight gain, usually within a week of loading. This technique typically gives a few layers of adsorbed water on the powder surface.2 These samples were then irradiated with γ rays. For Fe2O3 samples, the radiation yield of H2 as determined by the energy deposited in the total system, oxide and water, was measured as 0.04 ± 0.02 molecules/100 eV. Determination of the H2 with respect to the energy deposited in the water layer only, where the produced H2 originates, gives a value of 150 ± 60 molecules/100 eV. Compared to the known yield for bulk water, 0.45 molecules/100 eV,25 the yield in the oxide/water system is considerably higher. Previous work on CeO2, ZrO2, and UO2 found increased yields of 20, 150, 40 molecules/100 eV, respectively, from oxides with four or fewer water layers.2,3 These yields are too high to be due to anything but diffusion of energy from the oxide into the adsorbed water, leading to an apparent excess formation of H2. The exact nature of this energy transport is still uncertain but presumably involves an exciton migration from the bulk oxide to the surface waters.4 A hydrogen peak was not observed following the irradiations of FeO and Fe3O4. At the highest doses given, the H2 yields were estimated to be lower than 0.01 molecules/100 eV for energy deposited in the total system. This result suggests that the
The amount of water present on the oxide surface and its configuration is of importance to the measurement of H2 produced by radiolysis. From the TPD measurement for water, shown in Figure 3, there are at least two peaks for each
Figure 3. Temperature-programmed desorption curves for FeO (top), Fe3O4 (middle), Fe2O3 (lower) at a rate of 2 °C/min. Peaks are labeled with adsorption energies calculated using the Redhead method.
oxide powder. Multiple absorption sites can be expected since these are powder samples with many exposed faces and defects. The adsorption energy for each peak was calculated using Redhead’s method.21 For example, the Fe2O3 powder had peaks corresponding to adsorption energies of 1.2, 1.3, 1.6, and 1.8 eV. All of these energies are greater than the value expected for physisorbed water, which is 0.35 eV.22 This result suggests that there are multiple sites available for chemisorbed water bonding on all the oxides. Further information on these sites and how they will affect H2 production is unknown. The TPD method was used to determine molecules desorbed from the surface while a complementary technique, DRIFT, was used to analyze the water present on the oxide surfaces. Room temperature DRIFT spectra for FeO, Fe3O4, and Fe2O3 are shown in Figure 4 along with a reference spectrum for H2O ice.23 The Fe2O3 powder shows a wide adsorption band at 3400 cm−1, attributed to the OH stretching band, and a band at 1530 cm−1, due to the OH bending band, which are expected for adsorbed water.19,24 The peak at 2450 cm−1 is due to CO2 from the air environment that was present C
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The Journal of Physical Chemistry B physisorbed water layers observed using DRIFT spectroscopy on the Fe2O3 are critical for the production of H2. The interactions between all of the iron oxides and water are still of interest even if little or no water is absorbed on the FeO and Fe3O4. Observations of water−iron oxide radiolysis were performed by exposing slurries of oxide powder in 10 mM KBr solutions to gamma radiolysis. The addition of KBr is to eliminate reactions of the OH radicals produced in water radiolysis with the H2. Additional complications due to the presence of KBr are not expected because of its relatively low concentration. The slurries were varied by percent water as a function of mass with samples of 5, 10, 20, 40, 60, 80, and 90% water. Samples were then frozen, pumped under vacuum, and thawed to remove any air from the powder and glass tube. This cycle was performed three times, and then the sample tubes were sealed while frozen. To ensure that the samples remained mixed throughout the irradiation, the samples were rotated inside of the source at a speed of 10 rpm. Following gamma irradiation, the glass tubes were placed inside of Tygon tubing connected to the GC and were flushed with argon. The tubes were then cracked open, allowing any gases produced during irradiation to be analyzed with the GC. The yields of H2 for each of the oxides as a function of water percentage are shown in Figure 5. All H2 yields in Figure 5 are
A completely different response is observed for the FeO slurries than with those of Fe3O4 and Fe2O3. From about 40% to 100% water, the radiation chemical yields of H2 with FeO were found to slowly decrease almost linearly from about 7 molecules/100 eV to the value observed in bulk water (0.45 molecules/100 eV). These H2 yields are significantly larger than that observed with the other iron oxides. Below about 20% water, the yield of H2 increases rapidly with a decrease in water fraction up to 400 molecules/100 eV for the 5% water samples. Little or no H2 is observed for adsorbed water on FeO, but again that result could be due to the fact that little physisorbed water is on this oxide. A small amount of physisorbed water on FeO would probably lead to a large yield of H2 if the water was adsorbed. All oxides seem to indicate an excess production of H2 when very small amounts of water are near the water/oxide interface. More H2 seems to be produced by FeO than with the other two oxides, so oxidation of the iron may be a strong driving force. Slurries of the iron oxides were not examined with 4He ions because they could not be mixed satisfactorily. The ion beam is horizontal and only penetrates the slurry to a depth of a few tens of microns so essentially only the front face of the sample is irradiated. Any settling of the sample results in a bilayer with each layer being irradiated separately. γ Rays penetrate the entire sample, and a simple rotation of the entire sample container was sufficient to ensure a homogeneous distribution of the slurry. Water loading of 5% produces a system that behaves as one phase and could be examined with 4He ions. The yields of H2 with respect to the water are 1.82 and 1.02 molecules/100 eV for FeO and Fe2O3 irradiated with 5 MeV 4 He ions, respectively. These values can be compared to the yields of 400 and 0.65 molecules/100 eV found with γ rays. Clearly, there is a huge decrease in the production of H2 in 5 MeV 4He ion radiolysis compared to γ rays for FeO. Heavy ion radiation is expected to form defects in the bulk lattice, and these defects could be acting as traps for the migration of energy through the bulk to the surface to induce water chemistry. The 4He ion radiolysis of Fe3O4 was not successful because of a rapid decrease in the amount of water on the surface giving a marked decrease in H2 yields with time. The best estimate of the H2 yield in the radiolysis of Fe3O4 with 5 MeV 4He ions is 2.60 molecules/100 eV. This value is comparable to the corresponding 1.3 molecules/100 eV found with γ rays. Radiation Induced Changes to the Oxides. The oxide powders were analyzed before and after irradiation using XRD, XPS, and Raman spectroscopy. XRD was used to determine any changes to the bulk crystal due to radiation. By comparing the diffraction pattern for each sample to known structures in the International Centre for Diffraction Data (ICDD) Powder Diffraction File (PDF) database,26 the structures were determined. The FeO and Fe3O4 powders have a facecentered-cubic lattice, while α-Fe2O3 (hematite) has a rhombohedral lattice.6 Samples irradiated with γ rays and with 4He ions showed no noticeable change in the diffraction patterns following irradiation as shown by the XRD patterns displayed in Figure 6. The lack of variation in the patterns suggests that no gross modification to the bulk oxide structure occurs during irradiation or that any alterations to the bulk are relaxing in the time (hours) between irradiation and measurement. Chemical changes to the surface were evaluated using Raman spectroscopy. The spectra for FeO, Fe3O4, and Fe2O3 are
Figure 5. Radiation chemical yields of H2 as a function of the percentage of water in oxide−water slurries. The results for FeO are plotted on the right axis.
determined with respect to the energy absorbed only in the water layers for ease of comparison. The yields of H2 from Fe2O3 and Fe3O4 slurry samples are found to be nearly constant as a function of the amount of water present in the sample, with a value around 0.6 molecules/100 eV when calculated with respect to the energy deposited in the water. This value is only slightly higher and very similar to the yield for bulk water. The near invariance of the H2 yield with these two oxides suggests that there is little interaction between the oxide and bulk water to modify the amount of H2 produced. A slight increase in H2 yields are observed for both the Fe2O3 and Fe3O4 slurries at about 5% water. The above results with adsorbed water find that the yield of H2 increases to about 150 molecules/100 eV for Fe2O3. This result for adsorbed water on Fe2O3 would amount to a datum point at essentially zero water percentage on the plot in Figure 5. Such a response indicates that the enhanced H2 production occurs when only a few water layers are near to the oxide surface. Such a result was also observed for ZrO2 powders.19 D
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Figure 6. X-ray diffraction patterns for (A and B) FeO, (C and D) Fe3O4, and (E and F) Fe2O3 offset for clarity. The solid lines (A, C, and E) are for pristine powders taken from the bottle, and the dashed lines (B, D, and F) show samples after irradiation with 4He ions to a fluence of 1015 ions/cm2.
Figure 9. Raman spectra of Fe2O3 pristine powder taken from the bottle (solid line) and after irradiation with γ rays (dotted line) and 5 MeV 4He ions (dashed lines) to a fluence of 1015 ions/cm2.
the surface is responsible for some of the observed effects. The spectra are in agreement with previous work done on hematite, magnetite, and wüstite films,20,27−31 and the peak assignments are based on previous assessments by Jubb and Allen27 and de Faria, Silva, and Oliveira.20 The Fe2O3 samples, which have peaks at 227 (A1g), 247 (Eg), 293 (Eg), 411 (Eg), 423, 498 (A1g), 612 (Eg), and 1315 cm−1 (two magnon scattering), exhibit no changes in the Raman spectra following irradiation with 4He ions. This result suggests that the surface of the Fe2O3 samples remains stable during the irradiation. Only one peak at 652 cm−1 was observed with the FeO pristine sample. Following irradiation with 4He ions, islands of oxidized material could be observed on the particle surface through the Raman microscope. The bulk material gave a spectrum similar to the pristine FeO sample. The island regions gave spectra that more closely matched with the pristine Fe2O3 spectra. A similar buildup of islands was observed for the Fe3O4 sample. The pristine Fe3O4 sample had three peaks at 309 (T2g), 536 (T2g), and 665 cm−1 (A1g).20,27 Following irradiation with 4He ions, part of the sample had a spectrum resembling that of the pristine Fe3O4, and some parts more closely resembled the Fe2O3 spectrum. In addition, the areas of the surface that had peaks similar to the Fe2O3 spectra appeared to be red when viewed with the optical microscope on the Raman instrument, whereas the pristine Fe3O4 samples were silver gray in color. This observation indicates that some of the FeO and Fe3O4 powder oxidizes during 4He ion irradiation to Fe2O3. This oxidation could be due to any attached water on the surface or due to oxygen or water vapor in the air environment interacting with the oxide during irradiation. High-resolution XPS scans of the Fe 2p, O 1s, and C 1s regions were taken for each oxide sample before and after irradiation with 4He ions. Data analysis and calculations were done using PHI MultiPak,32 using relative sensitivity factors provided specifically for this instrument. The energy scale was calibrated by setting the main carbon peak, due to carbon contamination, for each sample as 285 eV and shifting the energy scale for the other regions accordingly. Atomic concentrations were calculated using peak areas and relative sensitivity factors are listed in Table 1. An examination of the reproducibility of peak areas using many different samples suggests that the uncertainty in the atomic concentration is around 5%. For all of the iron oxide powders, the atomic concentration of oxygen is higher than the expected value from the stoichiometry. This result suggests an oxygen-rich surface
shown in Figures 7, 8, and 9 for both the pristine sample and following 4He ion irradiation to a fluence of 1015 cm−1. No
Figure 7. Raman spectra of FeO pristine powder taken from the bottle (solid line) and after irradiation with γ rays (dotted line) and 5 MeV 4 He ions (dashed lines) to a fluence of 1015 ions/cm2. The lower spectrum for the 4He irradiated sample is from the bulk surface, while the upper spectrum is from the isolated islands.
Figure 8. Raman spectra of Fe3O4 pristine powder taken from the bottle (solid line) and after irradiation with γ rays (dotted line) and 5 MeV 4He ions (dashed lines) to a fluence of 1015 ions/cm2. The lower spectrum for the 4He irradiated sample is from the bulk surface while the upper spectrum is from the isolated islands.
variation in any of the oxides were observed up to doses of 1.5 MGy in gamma radiolysis, suggesting that mass displacement at E
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The Journal of Physical Chemistry B Table 1. XPS Measured Atomic Concentrations for Pristine, 5 MeV 4He Irradiated, and Gamma Ray Irradiated Iron Oxide Samples
FeO-pristine FeO−4He irradiated FeO-γ-ray irradiated Fe3O4−pristine Fe3O4-4He irradiated Fe3O4-γ-ray irradiated Fe2O3−pristine Fe2O3-4He irradiated Fe2O3-γ-ray irradiated
oxygen atomic concentration (%)
iron atomic concentration (%)
68 70
32 30
67
33
63 64
37 36
65
35
64 69
36 31
64
36
Figure 10. Normalized XPS spectra for O 1s peak for FeO powder. Spectra are offset for clarity. Peak analysis was performed using Phi MultiPak,32 using relative sensitivity factors provided specifically for this instrument.
and could be due to the presence of OH groups. Only the spectrum for the 4He ion radiolysis of Fe2O3 suggests a decrease in the concentration of oxygen fraction with irradiation. Much higher doses will be required to definitively show this phenomenon due to the relatively large uncertainty of the XPS measurements. The spectra for the Fe 2p high-resolution scans for FeO, Fe3O4, and Fe2O3 are not shown because no noticeable variation with irradiation was observed. Spectra for the Fe 2p transitions are given as Supporting Information. Due to the different chemical environments around the iron atoms in each of the three iron oxides, they should exhibit different binding energies for the iron core electron. Previous experiments have found the Fe(2p 3/2) binding energy for Fe2+ and Fe3+ atoms as 709.5 and 711.0 eV.33 However, when the spectra for the three oxides are compared, their spectra for Fe 2p appear identical. XPS measurements are estimated to sample only the top 10 nm of the surface due to the scattering of the detected low-energy electrons. This result suggests that FeO and Fe3O4 are surface oxidized to Fe2O3. McIntyre and Zetaruk also suggest that an overlayer of Fe2O3 forms on FeO and Fe3O4.33 In accordance with the XPS measurements, the different iron oxides look like Fe2O3 on the surface, but clearly their responses for the production of H2 are different, which suggest bulk features, such as efficiency of energy transport, are also factors. Somewhat similar results were observed when layers of different oxides were coated on SiO2.5 Comparison of the O 1s spectra for all of the pristine iron oxide samples in Figures 10, 11, and 12 show two visible peaks, one at ∼530 eV due to the oxygen in the oxide lattice and one at ∼532 eV due to the OH groups from the attached water molecules. The presence of the OH groups in the XPS spectra is consistent with the results from the TPD measurements in that both techniques show chemisorbed water present on all three oxide powders. Following gamma irradiation, there may be a slight increase in the OH peak area relative to the total peak area. The increase is especially noticeable with the 4He ion radiolysis; however, a closer analysis seems to indicate the formation of a new species at about 532.5 eV. Irradiation with 4 He ions was performed in an atmosphere of N2, and the lowresolution scans found atomic nitrogen on the surface following radiolysis. The passage of the 4He ions may be ionizing the N2 atmosphere, leading to surface attack. Gamma radiolysis occurs
Figure 11. Normalized XPS spectra for O 1s peak for Fe3O4 powder. Spectra are offset for clarity. Peak analysis was performed using Phi MultiPak,32 using relative sensitivity factors provided specifically for this instrument.
Figure 12. Normalized XPS spectra for O 1s peak for Fe2O3 powder. Spectra are offset for clarity. Peak analysis was performed using Phi MultiPak,32 using relative sensitivity factors provided specifically for this instrument.
at a much lower dose rate so this phenomenon is not observed. The new peak at 532.5 is thought to be due to a new O−N species, but further experiments will be required to give any more definitive information. The XPS analysis slightly indicates an increase in the OH fraction at the surface, but the evidence F
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authors thank the Center for Sustainable Energy at Notre Dame (cSEND) Materials Characterization Facilities for the use of the PHI VersaProble II X-ray Photoelectron Spectrometer, Jasco Micro-Raman Spectrometer NRS-5100, and Bruker D8 Advance Davinci Powder X-ray Diffractometer. Support of the National Science Foundation through MRI award 1126374 is acknowledged for the XPS data in this paper. The authors thank Prof. Michael Wiescher for making available the facilities of the Notre Dame Nuclear Structure Laboratory, which is supported by the U.S. National Science Foundation. The work reported here was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Basic Energy Sciences, Office of Science, United States Department of Energy through Grant DE-FC02-04ER15533. This contribution is NDRL-5049 from the Notre Dame Radiation Laboratory.
is not conclusive. There may be some sort of rearrangement of the water layers at the surface before XPS measurement. A major difference in the Raman spectroscopy and that of XPS is the area of the surface being examined. The observation site with XPS is about 100 μm and represents more of an average surface structure. However, the depth of examination is only about 10 nm with XPS. All the pristine oxides have virtually identical XPS spectra for the O 1s and Fe 2p, suggesting that the surface is Fe2O3 for all the samples. An area of only a few micrometers can be examined with the Raman microscope and surface nonhomogeneities are able to be observed. A Raman spectrum probes to a depth of micrometers so one observes bulk characteristics along with the surface. Further, more detailed observations will try to resolve the differences in these two techniques.
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CONCLUSIONS A variety of iron oxide powders with different amounts of associated water were irradiated using γ rays and 5 MeV 4He ions. A thorough characterization of the oxide surfaces was performed to determine variations due to radiolysis. Adsorbed water was characterized by both temperature-programmed desorption and diffuse reflection infrared Fourier transform spectroscopy to reveal a variety of active sites on the surface. The TPD results give more details about the specific chemisorbed water-binding sites. Molecular hydrogen production was found only from water adsorbed on Fe2O3, and the yield was several orders of magnitude greater than that of bulk water. Water does not seem to absorb well on the FeO and Fe3O4 surfaces, which could account for the nonobservable H2 production from these oxides. Aqueous slurries of FeO, Fe3O4, and Fe2O3 examined as a function of water fraction gave different yields of H2, depending on the oxide type and the amount of water. Little variation of H2 yields were observed for all of the oxides until very small amounts of water were associated with the powders. The results suggest that surfaceenhanced production of H2 occurs very near to the surface and involves only a few water layers. Examination of the iron oxide powders following irradiation by X-ray diffraction showed no change in crystal structure. Raman spectroscopy of the oxides revealed the formation of islands of Fe2O3 on the surfaces of FeO and Fe3O4. X-ray photoelectron spectroscopy of the oxides revealed the general formation of oxygen species following radiolysis, but details on the specific environment of the oxygen species are inconclusive.
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ASSOCIATED CONTENT
S Supporting Information *
Production of H2 in the gamma radiolysis of FeO with 5% water loading; total XPS spectra for Fe 2p in the pristine, gamma-irradiated, and 4He ion irradiated samples of FeO, Fe3O4, and Fe2O3; normalized XPS spectra for Fe 2p peaks for pristine Fe2O3 powder. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
(1) LaVerne, J. A. H2 Formation from the Radiolysis of Liquid Water with Zirconia. J. Phys. Chem. B 2005, 109, 5395−5397. (2) LaVerne, J. A.; Tandon, L. H2 Production in the Radiolysis of Water on CeO2 and ZrO2. J. Phys. Chem. B 2002, 106, 380−386. (3) LaVerne, J. A.; Tandon, L. H2 Production in the Radiolysis of Water on UO2 and Other Oxides. J. Phys. Chem. B 2003, 107, 13623− 13628. (4) Petrik, N. G.; Alexandrov, A. B.; Vall, A. I. Interfacial Energy Transfer during Gamma Radiolysis of Water on the Surface of ZrO2 and Some Other Oxides. J. Phys. Chem. B 2001, 105, 5935−5944. (5) Alam, M.; Miserque, F.; Taguchi, M.; Boulanger, L.; Renault, J. P. Tuning Hydrogen Production During Oxide Irradiation Through Surface Grafting. J. Mater. Chem. 2009, 19, 4261−4267. (6) Cornell, R. M.; Schwertmann, U. The Iron Oxides; Wiley-VCH: Weinheim, Germany, 2003. (7) Burns, W. G.; Marsh, W. R.; Walters, W. S. The λ IrradiationEnhanced Corrosion of Stainless and Mild Steels by Water in the Presence of Air, Argon and Hydrogen. Radiat. Phys. Chem. 1983, 21, 259−279. (8) Christensen, H. Effect of Water Radiolysis on Corrosion in Nuclear Reactors. Radiat. Phys. Chem. 1981, 18, 147−158. (9) Č uba, V.; Silber, R.; Múcǩ a, V.; Pospíšil, M.; Neufuss, S.; Bárta, J.; Vokál, A. Radiolytic Formation of Ferrous and Ferric Ions in Carbon Steel−Deaerated Water System. Radiat. Phys. Chem. 2011, 80, 440− 445. (10) Daub, K.; Zhang, X.; Noël, J. J.; Wren, J. C. Effects of γ-radiation Versus H2O2 on Carbon Steel Corrosion. Electrochim. Acta 2010, 55, 2767−2776. (11) Smart, N. R.; Rance, A. P.; Werme, L. O. The Effect of Radiation on the Anaerobic Corrosion of Steel. J. Nucl. Mater. 2008, 379, 97−104. (12) Vandenborre, J.; Crumière, F.; Blain, G.; Essehli, R.; Humbert, B.; Fattahi, M. Alpha Localized Radiolysis and Corrosion Mechanisms at the Iron/Water Interface: Role of Molecular Species. J. Nucl. Mater. 2013, 433, 124−131. (13) Dos Santos Afonso, M.; Di Risio, C. D.; Roitberg, A.; Marqués, R. O.; Blesa, M. A. Reductive Dissolution of Neutron- and GammaIrradiated Magnetite. Int. J. Radiat. Appl. Instrum., Part C 1990, 36, 457−460. (14) Lapuerta, S.; Millard-Pinard, N.; Moncoffre, N.; Bérerd, N.; Jaffrezic, H.; Brunel, G.; Crusset, D.; Mennecart, T. Origin of the Hydrogen Involved in Iron Corrosion Under Irradiation. Surf. Coat. Technol. 2007, 201, 8197−8201. (15) Lapuerta, S.; Moncoffre, N.; Bérerd, N.; Jaffrezic, H.; MillardPinard, N.; Crusset, D. Ion Beam Analysis of the Effect of O2 and H2O on the Oxidation of Iron Under Irradiation. Nucl. Instrum. Methods Phys. Res., Sect. B 2006, 249, 470−473. (16) Teodorescu, I.; Zberea, I. Influence of Reactor Irradiation on Surface Corrosion of Pure Iron. J. Nucl. Mater. 1967, 24, 347−349.
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS The authors congratulate Drs. John R. Miller and Marshall D. Newton on their body of science and wish them well. The G
DOI: 10.1021/jp510943j J. Phys. Chem. B XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry B (17) Basic Research Needs for Advanced Nuclear Energy Systems, Report of the Basic Energy Science Workshop on Basic Research Needs for Advanced Nuclear Energy Systems; U. S. Department of Energy: : Washington, D.C., 2006. (18) Garrett, B. C.; Dixon, D. A.; Camaioni, D. M.; Chipman, D. M.; Johnson, M. A.; Jonah, C. D.; Kimmel, G. A.; Miller, J. H.; Rescigno, T. N.; Rossky, P. J.; et al. Role of Water in Electron-Initiated Processes and Radical Chemistry: Issues and Scientific Advances. Chem. Rev. 2005, 105, 355−390. (19) Roth, O.; Dahlgren, B.; LaVerne, J. A. Radiolysis of Water on ZrO2 Nanoparticles. J. Phys. Chem. C 2012, 116, 17619−17624. (20) de Faria, D. L. A.; Silva, S. V.; de Oliveira, M. T. Raman Microspectroscopy of Some Iron Oxides and Oxyhydroxides. J. Raman Spectrosc. 1997, 28, 873−878. (21) Redhead, P. A. Thermal Desorption of Gases. Vacuum 1962, 12, 203−211. (22) Dyar, M. D.; Hibbitts, C. A.; Orlando, T. M. Mechanisms for Incorporation of Hydrogen in and on Terrestrial Planetary Surfaces. Icarus 2010, 208, 425−437. (23) Hudgins, D.; Sandford, S.; Allamandola, L.; Tielens, A. Mid-and Far-Infrared Spectroscopy of Ices-Optical Constants and Integrated Absorbances. Astrophys. J., Suppl. Ser. 1993, 86, 713−870. (24) Lavalley, J. C.; Bensitel, M.; Gallas, J. P.; Lamotte, J.; Busca, G.; Lorenzelli, V. FT-IR Study of the δ(OH) Mode of Surface Hydroxy Groups on Metal Oxides. J. Mol. Struct. 1988, 175, 453−458. (25) Pastina, B.; LaVerne, J. A.; Pimblott, S. M. Dependence of Molecular Hydrogen Formation in Water on Scavengers of the Precursor to the Hydrated Electron. J. Phys. Chem. A 1999, 103, 5841−5846. (26) PDF-4+ 2010 (Database); Kabekkodu, S., Ed.; International Centre for Diffraction Data (ICDD): Newtown Square, PA, 2010. (27) Jubb, A. M.; Allen, H. C. Vibrational Spectroscopic Characterization of Hematite, Maghemite, and Magnetite Thin Films Produced by Vapor Deposition. ACS Appl. Mater. Interfaces 2010, 2, 2804−2812. (28) Oh, S.; Cook, D. C.; Townsend, H. E. Characterization of Iron Oxides Commonly Formed as Corrosion Products on Steel. Hyperfine Interact. 1998, 112, 59−66. (29) Li, Y.-S.; Church, J. S.; Woodhead, A. L. Infrared and Raman Spectroscopic Studies on Iron Oxide Magnetic Nano-Particles and Their Surface Modifications. J. Magn. Magn. Mater. 2012, 324, 1543− 1550. (30) Bouchard, M.; Smith, D. C. Catalogue of 45 Reference Raman Spectra of Minerals Concerning Research in Art History or Archaeology, Especially on Corroded Metals and Coloured Glass. Spectrochim. Acta, Part A 2003, 59, 2247−2266. (31) Lin, T.-C.; Seshadri, G.; Kelber, J. A. A Consistent Method for Quantitative XPS Peak Analysis of Thin Oxide Films on Clean Polycrystalline Iron Surfaces. Appl. Surf. Sci. 1997, 119, 83−92. (32) PHI MultiPak, version 9; ULVAC-PHI, INC: Chanhassen, MN, 2011. (33) McIntyre, N. S.; Zetaruk, D. G. X-ray Photoelectron Spectroscopic Studies of Iron Oxides. Anal. Chem. 1977, 48, 1521− 1529.
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DOI: 10.1021/jp510943j J. Phys. Chem. B XXXX, XXX, XXX−XXX
