Article pubs.acs.org/JPCC
Radiolysis of Water on ZrO2 Nanoparticles Olivia Roth,† Bjoern Dahlgren,† and Jay A. LaVerne*,†,‡ †
Radiation Laboratory and ‡Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, United States ABSTRACT: The radiolysis of water adsorbed on the surface of ZrO2 nanoparticles has been examined using a variety of spectroscopic techniques. Irradiations were performed with fast electrons, γ rays, and 5 MeV helium ions. Infrared spectroscopic analysis of the surface reveals little change in the surface stoichiometry, which probably indicates the relative insensitivity of this technique to the doses given. X-ray photoelectron spectroscopy reveals a loss in the relative number of terminal OH groups on the surface with radiolysis. The maximum production of excess H2 is found when a single water layer or less is adsorbed on the terminal OH surface layer. Little or no H2 seems to come from the surface-bound OH groups.
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INTRODUCTION Zirconia (ZrO2) is a versatile ceramic compound used in a variety of applications, including, for example, catalysis,1 medical implants,2 and electronics,3 and yet some of the fundamental chemical processes occurring at its surface are not well-understood. Of special importance is the radiation-induced chemistry at the water−ZrO2 interface that occurs in the nuclear industry. Zircaloy, a zirconium alloy, has chemical and physical properties suitable for many applications in the nuclear industry, and it is commonly used as cladding material on the fuel rods in nuclear reactors.4 The passive surface of zircaloy is ZrO2, and during the lifetime of a nuclear reactor, the fuel rods and other components will be exposed to high levels of radiation. Understanding the radiation-induced processes, in particular, the chemistry at the water−solid interface, is a key issue in the safety and performance assessment of nuclear reactors. A variety of recent studies have shown that the radiolysis of water at the interface of solid nanoparticles is very interesting from a fundamental science aspect.5−11 Water exposed to ionizing radiation will decompose into a number of radical and molecular products (eaq−, H•, OH•, HO2•, H2O2, and H2).12 Solid surfaces often alter the yield of products from water radiolysis as compared to the bulk liquid. For instance, pulse radiolysis studies of aqueous silica suspensions have found increased formation of hydrated electrons,13 and increased yields of H2 have been observed in the γ radiolysis of water and ZrO2 particles.5,6 However, no complementary oxidizing species has been observed in the aqueous phase. Pulse radiolysis studies seem to indicate that, while electrons escape from the particle, the holes remain on the surface of the particle.14 Experiments with very low amounts of water on ZrO2 surfaces have shown that much of the interfacial radiation chemistry occurs very close to the surface, indicating the involvement of the chemisorbed water.6,15 Adsorption of water on many ceramic oxides occurs through dissociation, resulting in the formation of OH groups on the surface.16,17 These OH © 2012 American Chemical Society
groups can be identified and studied with infrared spectroscopy. Two characteristic types of OH groups have been identified on ZrO2 surfaces: terminal Zr−OH (3760 cm−1) and bridged Zr− OH−Zr (3660 cm−1).18,19 These OH surface groups are expected to play an important role in the radiolysis of water with nanoparticles. The aim of this work is to elucidate the role of surface OH groups in the radiolysis of water adsorbed on ZrO2 surfaces. This effort has been accomplished by infrared studies of the surface and simultaneous determination of H2 yields following irradiation of water on ZrO2 in well-controlled environments. The amount of adsorbed water has been modified by varying the temperature. Both diffuse reflection infrared Fourier transform (DRIFT) spectroscopy and temperature-programmed desorption (TPD) studies have been used to correlate the temperature to the amount of adsorbed water. X-ray photoelectron spectroscopy (XPS) analysis of the surface following irradiation was used to reveal radiation modifications to the surface.
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EXPERIMENTAL SECTION Irradiations. Fast electron radiolysis was performed with a High Voltage Engineering single-ended KN Van de Graaff in the Notre Dame Radiation Laboratory. The beam was delivered in a continuous mode at an energy of 2.8 MeV. A collimator was used to give a beam diameter of about 3 mm. Beam current was typically about 20 nA incident to the sample and monitored with a quartz fiber optic light guide. Irradiations were performed up to a fluence of about 2 × 1015 electrons/ cm2. Heavy ion irradiations were performed in the Tandem FN Van de Graaff facility of the Notre Dame Science Laboratory in the University of Notre Dame Physics Department. Experiments were performed with 5 MeV 4He2+ ions collimated to Received: May 2, 2012 Revised: July 17, 2012 Published: July 18, 2012 17619
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performed with empty sample cells and subtracted from those with added powder to eliminate contributions from the background. X-ray photoelectron spectroscopy (XPS) studies were performed using a Kratos XSAM 800 with an Al Kα X-ray source (1486.6 eV) and a 90° takeoff angle. Powdered samples were pressed into double-sided conductive carbon tape attached to the sample stubs. All peaks were fitted with Gauss-Lorentz peaks using the Kratos Vision 2 software to obtain peak area information. A linear baseline was used in the fitting processes. Energy determination was relative to the C 1s peak at 285 eV.
about 3 mm in diameter. This energy was incident to the sample with energy loss to windows determined using standard stopping power compilations.20 Ion beam currents were typically 10 nA charge, and fluencies up to 5 × 1014 ions/ cm2 were used. The irradiation cell used for both the fast electron and the He ion irradiations was a modified high-temperature reaction chamber manufactured by Harrick Scientific Products. This chamber allows a powder sample of about 100 mg to be placed in a cup and be completely sealed with a dome containing three windows. The sample could be heated to at least 500 °C, and water cooling maintained the outside of the chamber at ambient. The dome contained two KBr windows (2 mm × 15 mm) for infrared probing. A third window in the dome was a thin microscope cover glass 15 mm in diameter and 170 μm thick (39.9 mg/cm2) for beam entrance. An inlet and outlet port connected the reaction chamber to a gas chromatograph and allowed purging of the dome through the sample. Materials. Chemicals were used as received without further purification. Argon used for purging was of ultrahigh purity. The ZrO2 (99.9%) powder was obtained from Alfa Aesar. The specific particle surface was 3.1 m2/g as determined with a Quantachrome Autosorb 1 surface area analyzer using the BET (Brunauer−Emmett−Teller) method. This area corresponds to a perfect sphere of about 350 nm. The ZrO2 was baked at 500 °C for 48 h prior to the experiment to remove any hydrocarbon contaminants. Baking was not found to alter the particle surface area. Analysis. The sample was placed in the reaction chamber, sealed, brought to the appropriate temperature, and flushed prior to irradiation with fast electrons or He ions. After irradiation, the entire reaction chamber was immediately transferred to a Bruker Vertex 70 diffuse reflection infrared Fourier transform (DRIFT) spectrometer for analysis. A praying mantus diffuse reflection accessory by Harrick Scientific Products was used to couple the high-temperature reaction chamber with the spectrometer. The DRIFT spectrometer is equipped with a DTGS detector and is controlled by the OPUS software. The sample chamber was flushed with argon after each irradiation, and the outlet gas was analyzed for H2 using gas chromatography. Ultra-high-purity argon was used as the carrier gas with a flow rate of about 50 mL/min. The argon passed through a constant flow regulator, an injection septum, and a four-way valve and into a 3 m 5× molecular sieve column of a SRI 8610C gas chromatograph with a thermal conductivity detector. The four-way valve was used to connect the chromatograph to the reaction chamber. Calibration of the gas chromatograph was performed by injecting pure H2 with a gastight microliter syringe. The error in gas measurement is estimated to be about 5%. Temperature-programmed desorption (TPD) studies were performed with the same reaction chamber as used in the radiolysis. The dome was removed, and the reaction chamber body was coupled to a high-vacuum chamber containing a Pfeiffer Prisma residual gas analyzer. The base pressure in this system was 10−7 Torr. Samples were prebaked at 400 °C for 48 h to remove any contaminants. Water desorption was monitored at mass 18 while the temperature was ramped at rates of 2−10 °C/min with a Watlow 989 temperature controller. The temperature of the desorption peak maximum was used to determine the water energy of adsorption using the Redhead method with a frequency of 1013 s−1.21 Runs were
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RESULTS AND DISCUSSION Water on ZrO2. A comparison of the infrared spectra of water on the surface of ZrO2 nanoparticles with that for amorphous water ice is shown Figure 1a.22 Room-temperature
Figure 1. (a) Infrared spectra of ZrO2 nanoparticles at room temperature (dashed line) and 100, 200, 300, and 400 °C (solid lines), and for amorphous ice, ref 22. (b) Temperature-programmed desorption of water on ZrO2 nanoparticles over the same temperature range. The IR spectra for amorphous ice and room-temperature ZrO2 are offset for clarity.
ZrO2 was examined with the powder in equilibrium with water at 53% relative humidity (approximately 0.1 wt % loading of water). The main infrared features in common with water ice and adsorbed water are the OH stretch at about 3300 cm−1 and the OH bend at about 1650 cm−1. A broad band below 1000 cm−1 in the ZrO2 is due to bulk absorption. Water adsorbed at 17620
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room temperature shows a broad OH stretch due to the relatively large amount of chemisorbed water. This band for adsorbed water is shifted to slightly higher energies than for amorphous ice. A relative humidity of 53% represents about 1.4 layers of adsorbed water.6 Heating the sample results in the loss of water layers and a corresponding decrease in the peak due to the OH stretch. At 400 °C, most of the chemisorbed water is gone and the prominent peaks are due to the OH groups: terminal Zr−OH (3760 cm−1) and bridged Zr−OH−Zr (3660 cm−1).18,19 Smaller peaks at 2350 and 2900 cm−1 are due to the absorption by components in air. The sample is enclosed in argon, but the beam passes through air in the spectrometer compartment. Adsorbed water is always in equilibrium with the surrounding water vapor, and the temperature at which the water preferentially desorbs can be used to estimate the energy of adsorption. This energy gives a measure of the binding or organization of the water on the surface. Nanoparticle surfaces may have a variety of exposed crystalline sites and defect sites that will affect the adsorption of the water layers, and the results may be very different than those found with single crystalline faces commonly examined using material science techniques. An examination of the TPD obtained for the nanoparticles used in this work reveals two broad chemisorbed water bands with peaks at about 147 and 225 °C (Figure 1b). This loss of water corresponds to the decrease in the infrared absorption peak at 3300 cm−1. Within the temperature range of 100−400 °C, several water layers exist on top of the OH groups bound to the ZrO2 surface. The peak maximum at about 147 °C corresponds to an adsorption energy of about 138 kJ/mol (1.4 eV) when determined by the Redhead method.21 The peak at 225 °C has a much higher energy of 164 kJ/mol (1.7 eV). Weakly bound physisorbed water is assumed to have an adsorption energy of less than 0.35 eV.23 Such a low energy would correspond to a temperature of about −163 °C in Figure 1b, which is clearly below the range used in this work. Adsorption energies of over 1 eV observed in this work suggest that the chemisorbed water is very strongly bound and must be an integral part of the surface through strong network bonding with the terminal OH groups. The IR spectra clearly show the OH groups at 400 °C, so they must be released at even higher temperatures. The reported TPD of water on ZrO2 using a high-vacuum chamber shows a sharp peak at about −30 °C due to the loss of physisorbed water, followed by a broader peak at 120 °C due to the loss of the first layer of chemisorbed water.5 The chemsorption peak at about 120 °C corresponds well with the lowest-temperature peak observed in this work. However, that work did not observe anything else at higher energies. Both studies observe two peaks in the TPD, but the temperature shift is over 100 °C, which is far too large to be due to experimental artifacts. Variation in the temperature ramp up to a rate of 10 °C/min still gave two peaks, although the highest-temperature peak was obscured when flash desorption became prevalent. The presence of a second TPD peak at higher energies is sometimes attributed to the trapping of water in pores or other confined geometries on the nanoparticle surface.23 However, the nanoparticles used here are not expected to be porous, and the second peak at higher energies probably corresponds to water very strongly bound to the surface, possibly through hydrogen bonding with the OH groups on the surface. Particle surface characteristics can be further characterized using standard adsorption techniques with an inert gas. Adsorption/desorption isotherms of nitrogen on the ZrO2
particles used here are shown in Figure 2. These particles exhibit the standard Type II isotherm characterized by a large
Figure 2. Adsorption (red circle) and desorption (blue square) isotherms for N2 on ZrO2 nanoparticles.
BET “C” constant. The lack of any major hysteresis between the adsorption and desorption indicates normal monolayer-tomultilayer adsorption with no significant porosity of the surface.24 Although the isotherm was performed with nitrogen, a similar response is expected with water vapor.24 Previous studies on the adsorption of water on ZrO2 have determined that there are about 10 OH groups/nm2.25,26 Furthermore, the first water layer on the dissociated OH water layer is not localized, but very strongly bound to the OH layer by hydrogen-bonding networks.24,25 The conclusions of the TPD and BET studies are that the first water layer is very strongly bound to the OH-terminated surface and a second chemisorbed layer also exists at room temperature. An increase in the relative humidity is expected to lead to the deposition of physisorbed water layers on top of the chemisorbed ones. Fast Electron Irradiation. Samples of water on ZrO2 nanoparticles were prepared at temperatures from ambient to 400 °C. DRIFT spectra were taken before and after irradiation with fast electrons. Figure 3 shows the DRIFT spectra for fast electron radiolysis at 400 °C. Virtually no change in the spectrum is observed following irradiation. A very slight
Figure 3. Infrared spectra of ZrO2 nanoparticles before (dashed line) and after (solid line) γ irradiation at 400 °C. 17621
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increase in absorbance can be observed for the OH stretch at 3300 cm−1, which may even indicate the formation of water. Rearrangement of the structured OH groups to the lessstructured adsorbed water could account for this phenomenon, but a corresponding loss in the absorption of the OH groups is not observed. The yield of O2 is not observed in the radiolysis of water on ZrO2 nanoparticles.6 Interestingly, the formation of infrared peaks due to HOO groups is not observed on the surface, which would account for the residual O atoms following H2 formation. Either the doses are not sufficient to form observable amounts of these species or they decompose before measurement or the residual O atoms are being incorporated into the bulk solid and masked by the broad peak below 1000 cm−1. Most of the energy deposited in this system is initially in the ZrO2 phase because it makes up about 99.9% of the total electron density of the system. Previous studies have suggested that a substantial amount of the energy in the ZrO2 passes through the interface and into the water phase.5,27 This energy must pass through the OH groups on the surface to the adsorbed water layers. Either these OH groups are very radiation-resistant and the energy passes through them without damage or they are reformed from the dissociation of residual water on the time scales of these experiments or the infrared response is not sensitive enough at these doses. Previous studies on the adsorption of water on ZrO2 have determined that there are about 10 OH groups/nm2 or about 3.1 × 1019 OH groups per gram of the material used in this work.25,26 The corresponding reflectance at 3300 cm−1 is only about 0.16, so a significant fraction of the surface would have to be modified to detect an infrared change. No variation in the infrared spectra with radiolysis was observed at the lower temperatures. However, examination of the underlying OH groups is more difficult at lower temperatures because of the broad OH stretch due to the chemisorbed water, and variation in the amount of surface OH groups could be occurring due to radiolysis. Molecular hydrogen was determined in conjunction with each of the DRIFT spectra at the various temperatures. Absolute measurements could not be performed in the present configuration, but previous results show that the yield of H2 in γ radiolysis is about 0.08 molecules/100 eV for energy deposition determined with respect to the total system: water and ZrO2. Relative yields for H2 formation are shown in Figure 4 for fast electron radiolysis. With increasing temperature, there is a significant increase in H2 production until about 200 °C, followed by a decrease at higher temperatures. Virtually no H2 is observed for radiolysis at 400 °C, where the system is essentially composed of surface OH groups. Again, the OH groups on the surface seem to be relatively radiation inert, in agreement with no significant changes being observed in the DRIFT spectra at 400 °C on irradiation. The formation of H2 near the interfaces is consistent with the previous work of Petrik et al., who also observed a sharp drop in the relative amount of H2 produced with increasing desorption of the chemisorbed water layer.5 That work did not measure the production of H2 at the higher temperatures, but relied on preheating to drive off the adsorbed water layers, followed by radiolysis at room temperature. The similarity in the results between the two techniques also suggests that the number of water layers, and not temperature, is affecting the production of H2.
Figure 4. Relative H2 yields as a function of temperature in the radiolysis of water on ZrO2: (red circle) 2.8 MeV electrons and (blue square) 5.0 MeV He ions. The right-hand side shows an estimate of the number of intact water layers (black triangle) on the surface.
The enhanced yields of H2 at about 200 °C corresponds to just a small amount of chemisorbed water on the surface OH groups, as shown by the DRIFT and TPD results. Variation in the water absorption peak at 3300 cm−1 can be used to approximate the amount of chemisorbed water remaining on the surface by assuming no water overlayer remains at 400 °C in combination with the previously measured amount of water at a relative humidity of 53%.6 The resulting amount of intact water on the surface is shown in Figure 4. Approximately half of the surface is covered at 200 °C, where maximum production of H2 occurs. Apparently, this small overlayer of chemisorbed water gives the best configuration for enhanced H2 formation. Additional water on the ZrO2 tends to give results more similar to the radiolysis of bulk water. Previous EPR studies identified the existence of H atoms on the surface of irradiated water on ZrO2.15 H atoms are likely precursors to H2 by abstraction reactions with the water. If the OH surface groups are the source for H atoms, then these OH surface groups are reformed on short time scales. H atoms may be more mobile with a few layers of chemisorbed water, or a few layers of chemisorbed water is a more efficient source of the H atoms than multiple layers. The present set of results is still not conclusive on the mechanism of enhanced H2 formation from water on ZrO2 nanoparticles. XPS gives a direct measurement of the binding energies of the various electrons at the surface of a material. Since electron energy levels are sensitive to the environment, the results can be used to examine surface chemical entities. The technique is often used for observing the variation in surfaces used in catalysis, including the oxidation of zirconium.28 Typical energies for the various electrons examined in the XPS of ZrO2 are the Zr 3p electrons at about 333 and 346 eV and the Zr 3d electrons at 181 and 183 eV. The O 1s electrons from the ZrO2 are observed at 532 eV. Small amounts of O 1s electrons from the surface OH groups are observed at 535 eV. Figure 5a shows a typical XPS spectrum of the O 1s electrons. No variation in the binding energies of any of the bonds is observed in the radiolysis with γ rays to a dose of 100 kGy. There is also no apparent formation of any new bonds upon irradiation to this dose. The relative magnitudes of the Zr electrons remain unchanged by radiolysis, but there is a significant change in the relative amount of O 1s electrons. Figure 4 shows that the 17622
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observed with fast electron radiolysis. The relative increase with He ions is smaller than that observed with fast electrons. Previous studies determined the absolute yield of H2 to be about 0.017 molecules/eV of energy deposition, determined with respect to the total system: water and oxide.6 This value is considerably lower than that observed with fast electrons, which may explain the smaller relative yield observed at 200 °C. Even though the yield of H2 may be smaller with 5 MeV He ions than with fast electron or γ radiolysis, the results presented here suggest that the mechanism for H2 formation is essentially the same for both types of radiations. The He ion irradiation of ZrO2 nanoparticles resulted in a change in the XPS spectra of the O 1s electrons similar to that observed with γ rays. There was a very noticeable decrease in the relative amount of OH electrons. A considerable amount of energy is deposited directly to the interface by the passage of the He ions, which may be difficult to distinguish from energy migrating from the bulk of the nanoparticle.
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CONCLUSIONS The surface OH groups formed by the adsorption of water on ZrO2 seem to be relatively radiation inert on the long time scale, as observed in the infrared. Much of the dissociation of the O−H bond by transport of energy through the interface is quickly repaired. XPS analysis of irradiated surfaces does show a slight decrease in the relative amount of OH groups on the surface. The production of H2 seems to be most efficient with one or fewer water layers on top of the surface OH groups. Little H2 is produced when the surface consists of only OH groups. The exact mechanism for the formation of H2 is still uncertain.
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Figure 5. XPS spectra of the O 1s electrons from ZrO2 (a) unirradiated and (b) irradiated with γ rays to 100 kGy.
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
relative amount of O 1s electrons in the OH groups decreases from 18.4% to 6.4% upon irradiation with γ rays to 100 kGy. Apparently, some of the surface OH groups can be modified by radiolysis. No new peaks are observed that could be attributed to peroxides, so the loss of OH groups must be accompanied by the formation of bulk bound O atoms. He Ion Irradiation. Experiments were performed with incident 5 MeV He ions under identical conditions as with the fast electron radiolysis. These He ions mimic irradiations with α particles, which is the predominant decay mode for the transuranic elements. The DRIFT spectra taken at the various temperatures are not shown, because they are virtually identical to those found with fast electron radiolysis. No significant variation in the DRIFT spectra is observed with He ion radiolysis at all of the temperatures examined here. He ions have a linear energy transfer (LET = −dE/dx) of 93 eV/nm as compared with about 0.2 eV/nm for fast electrons. The large local deposition of energy is expected to lead to significant changes in the kinetics due to the increased probability of second-order reactions in the ion track. In addition, high LET can affect the formation of the exitons and even modify the crystalline structure.29,30 However, even at these relatively high values of LET, no observable change in the IR spectrum of the ZrO2 surface is observed. The production of H2 in the He ion radiolysis of water on ZrO2 nanoparticles was observed in conjunction with measurement of the DRIFT spectra. Relative H2 yields with He ions are shown in Figure 4 as a function of temperature. An increase in H2 production is observed at about 200 °C, as similarly
ACKNOWLEDGMENTS The authors thank Prof. Michael Wiescher for making the facilities of the Notre Dame Nuclear Science Laboratory available to us. The Nuclear Science Laboratory is supported by the U.S. National Science Foundation. The research described herein was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through award DE-FC0204ER15533. This contribution is NDRL-4924 from the Notre Dame Radiation Laboratory.
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