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2009, 113, 9436–9439 Published on Web 05/11/2009
Intense Atomic Oxygen Emission from Incandescent Zirconia Katsuro Hayashi,*,† Tetsuya Chiba,† Jiang Li,‡,§ Masahiro Hirano,‡,§ and Hideo Hosono†,‡,§ Materials and Structures Laboratory, Frontier CollaboratiVe Research Center, Tokyo Institute of Technology, and ERATO-SORST, Japan Science and Technology Agency, Yokohama 226-8503, Japan ReceiVed: March 10, 2009; ReVised Manuscript ReceiVed: April 28, 2009
Particle emissions from an oxide-ion conducting solid electrolyte (tetragonal zirconia polycrystal, 3% Y2O3doped ZrO2) have been examined using a high vacuum evaluation system. Oxygen gas with a pressure range from 10 to 3 × 103 Pa was fed into a 2 mm outer diameter zirconia tube, whose central region of 10 mm length was heated to 1400-1800 °C by direct Joule heating. The emission species were identified by appearance potential spectroscopy using a quadrupole mass spectrometer. Atomic oxygen (AO) is the dominant species that is radiantly emitted from the high-temperature region. Temperature dependence of the AO emission intensity exhibits far less activation energy (∼0.3-1.0 and 0.7 eV, on average) than electric field-assisted thermal O- ion (∼2 eV) and electron (∼6 eV) emission from zirconia reported before. High AO emission flux (in the order of 1017 atoms · cm-2 · s-1) is promising for use as practical AO sources. Understanding thermal desorption of oxygen species from fast oxide ion conductors with fluorite structures, including zirconia (ZrO2),1 ceria (CeO2),2 and their solid solutions,3 is essential for their use in many applications including oxygen buffers for automobile catalytic converters, partial oxidation catalysts for hydrocarbon reforming, and solid oxide fuel cells. Thermal desorption from solid surfaces itself is a central interest in surface science as well as being a principal method for probing surface phenomena. If we consider the surfaces of noble metals, for example, the energetics and kinetics of molecular oxygen (MO) adsorption, the dissociation to atomic oxygen (AO) and its desorption have been thoroughly investigated.4 However very little research has been performed in the desorption of AO originated from oxygen species incorporated in, or permeated through, solids. Manipulation of these phenomena would also lead to techniques for generating reactive oxygen species, selectively. In this context, electron stimulated desorption (ESD) of AO from zirconium-doped silver film,5 ESD of O+ ion from ceria,6 and thermal O- ion emission from zirconia,7,8 ceria,9 and calcium aluminates10,11 have been reported. These studies have been performed in the temperature range of ∼400-800 °C. Behavior at much higher temperature ranges is not only interesting in itself but is also expected to reveal an unknown process due to its limited kinetics at lower temperature. In this study, the emission properties of oxygen species from incandescently heated (1400-1800 °C) 3% yttria (Y2O3)-doped zirconia in vacuum have been investigated. We report here an intense emission of AO originating from oxygen incorporated into the zirconia. * To whom correspondence should be addressed. E-mail: k-hayashi@ lucid.msl.titech.ac.jp † Materials and Structures Laboratory. ‡ Frontier Collaborative Research Center. § ERATO-SORST, Japan Science and Technology Agency.
10.1021/jp902159p CCC: $40.75
Figure 1. Experimental geometry. (a) Structure of the emitter. Two gas-permeable internal electrodes consisting of a 87Pt-13Rh alloy thin wire and La0.9Sr0.1CrO3-δ ceramics were formed with a 10 mm gap inside a polycrystalline 3% Y2O3-doped ZrO2 ceramic tube with 2.0 mm in outer and 1.2 mm in inner diameters. The inner tube is hermetically sealed from the outside using graphite sealants and stainless steel connectors that are electrically connected to the internal electrodes for the electricity supply. Oxygen gas was supplied from one connector. The 87Pt-13Rh alloy spiral wire was used as an auxiliary heater or as an extraction/suppression electrode for charged particles. (b) Operation in ambient air. (c) Operation in vacuum. Oxygen gas of 3.0 kPa was supplied inside the ceramic tube. The temperature of the hot zone was 1700 °C, which was measured with a radiation thermometer. The oxygen partial pressure in the vacuum chamber was 5 × 10-4 Pa.
The experimental setup is illustrated in Figure 1a. Two gaspermeable electrodes separated by a 10 mm gap were formed in a polycrystalline zirconia tube with a 2 mm outer diameter. The inner tube was hermetically sealed from the outside and then fed with oxygen gas at a pressure ranging from 10 to 3 × 103 Pa. The zirconia tube was preheated by heat radiation from 2009 American Chemical Society
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Figure 2. Detection of atomic oxygen (AO). (a) Ionization electron energy dependence of the intensity at a mass-charge ratio (m/e) of 16 (O+). The hot zone temperature was 1700 °C and the inner oxygen pressure was 3.0 kPa. Data were collected when the inlet of a quadrupole mass spectrometer was moved to the “close” position 20 mm from the hot zone (solid line) and also collected when the inlet was moved to the “distant” position 100 mm away from the hot zone and a shutter inserted between them (dashed line). The data is normalized so that intensities at more than 40 eV coincide. (b) The hot zone-to-detector distance dependence of the intensities for AO and molecular oxygen (MO). The hot zone temperature was 1800 °C, and the inner oxygen pressure was 3.0 kPa. AO was detected at m/e ) 16 (O+) and an ionizing electron energy of 18.0 eV whereas MO was detected at 32 (O2+) and 70.0 eV. The data is normalized to that at 20 mm. The inset is a logarithmic plot.
a spiral metal wire wound around the tube to increase its electrical conductivity sufficient for direct Joule heating. Then an AC current was applied to the electrode gap in the zirconia tube (the hot zone), heating it to a temperature between 1400 and 1800 °C (Figure 1b,c). After the stabilization of the temperature, the electricity supply to the spiral wire was turned off. The power required to maintain these high temperatures was only a few tens of watts because of the low thermal conductivity of zirconia. The AO emission was evaluated in a vacuum chamber with a background pressure better than 1 × 10-4 Pa using a Hiden PSM-003 quadrupole mass spectrometer (QMS). The inside of the spectrometer was differentially pumped to a pressure ∼40 mm and is insensitive to the shutter insertion. The MO signal thus originates mainly from those reside inside the chamber. Figure 3 displays the dependence of AO and MO intensity on the hot zone temperature and the feed oxygen pressure inside the tube. The QMS only provides relative intensity data. The absolute emission density of AO from the hot zone (Figure 3a,b) was estimated from the oxidation rate of a silver film deposited on a quartz-crystal microbalance16 (see Supporting Information, S2). The absolute MO evacuation rate (Figure 3c,d) was calculated from the oxygen partial pressure in the chamber (10-4 to 10-3 Pa) and the effective pumping speed (∼100 L s-1). The temperature and the oxygen pressure dependence of AO and MO intensities are similar, suggesting that most MO originates from a recombination of AO. Furthermore, the evacuation rate of MO corresponds to only ∼1/2-1/5 of the total AO emission rate. This value provides an upper boundary for the MO/AO emission ratio since not all of the emitted AO is consumed by surface oxidation in the inner chamber, but recombines to form MO. Hence, the observed small MO/AO ratio may indicate that very little, if any, emission of MO originates from the zirconia surface. The AO emission intensity is enhanced by increasing the inner oxygen pressure (Figure 3a). This dependence suggests that the AO emission originates from oxygen species diffusing through the zirconia tube, which is driven by the
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difference in oxygen chemical potential between the inner and outer surfaces of the zirconia. If the AO generation on the outer surface and the supply of O2 on the inner surface were determined by the equilibrium of O2 h 2O0, the intensity-oxygen pressure characteristics would show a power dependence with a slope of 1/2 in log-log coordinates. The observed characteristics in Figure 3a generally follow this 1/2 dependence. However, the value of the slope in fact varies between ∼1/4 and ∼1 depending on the operating conditions, together with a significant apparent activation energy change (∼0.3-1.0 and 0.7 eV, on average) in the Arrhenius plot. This complicated behavior likely suggests a presence of crossovers among the rate determining steps for the AO emission. Their candidate processes will be described later. The value of the activation energy in the temperature dependence of emission intensity is considerably smaller than that observed for thermal O- ion (∼2 eV)7,8 and electron emissions (∼6 eV)17 from zirconia. To examine the effect of electric fields, the internal zirconia electrodes were negatively biased against an electrically grounded metal spiral wire. We observed a weak negative charge emission that consisted of electrons in the majority (>95%) and O- ions in the minority. The emission current density was a few tens of µA cm-2 under a potential of up to 150 V (an average electric field of 1.5 kV cm-1). The negative particle flux was at least a few orders of magnitude less than that of AO, which is equivalent to 2-20 mA cm-2. Furthermore, the negative particle emission was completely suppressed by applying a positive potential higher than the peak voltage of the AC power supply. In contrast, the AO emission intensity was not affected appreciably by the potential (see Supporting Information, S3). These observations suggest that the AO is generated via a process that is not directly related to electron emission in vacuum. To deduce the AO desorption process, we first employ the general scheme for oxygen incorporation and desorption on the gas-metal oxide biphase boundary18-21 2O2 gas h O2 ads h O2-ads h O22-ads h Oads or LC h OLC h 2Olat
(1)
where subscripts indicate the gas phase, adsorbent, low coordinate surface site, and lattice sites. The O22- ion on the surface is energetically unfavorable and even its general presence is controversial. Hence the interconversion between O2- and Oions is generally a candidate for the rate-determining step of oxygen incorporation and liberation.21 The formation of AO may be initiated by a preferable hole trap of O2- ions at low coordinate surface sites around defects such as step edges and vacancies, forming O- ions. Coalescence of two O- ions may then produce O22- according to the above scheme (1). However, this process may be inhibited by a high-energy barrier. Instead, we suggest that possible next steps are a disproportionation of two O- ions to generate an AO and an O2- ion (2O-ads or LC f O2-ads or LC + Ogas), or a sequential oxidization of O- ion to AO, followed by AO desorption from the surface. Another possible process is that related to the formation of interstitial AO in the ZrO2 bulk as suggested in an experimental study on Si oxidation via an undoped-ZrO2 film22 and a theoretical study on undoped-ZrO2.23 The theoretical calculation predicts that the ground state AO is incorporated exothermally into the lattice with a concurrent energy gain of 0.8 eV, virtually forming an O22- ion on the oxygen lattice site. Thus, the minimum energy barrier for AO emission in vacuum from the lattice O22precursor is expected to be only 0.8 eV. Interestingly, this value
is comparable to that observed in our experiment. Further investigation using theoretical calculations may be a key to understanding the mechanism of direct AO desorption. The AO is well known as a key species that plays important roles in combustion, plasma oxidation, photosensitized reactions, etc. Probably the most important field relevant to the AO is vacuum technology.5,16,24,25 An instrument that produces a high flux of oxygen atoms would be useful in many application areas including low-temperature formation of ultrathin oxide layers on semiconductors,26 thin film fabrication of novel oxide materials,24,27 and surface cleaning and modification in conjunction with surface analytical techniques. Our method has advantages over the conventional plasma discharge in its wider operation pressure range and the suppression of energetic ionic species that potentially damage target materials. The maximum emission flux from the zirconia surface was in the order of 1017 atoms cm-2 s-1 and was quite high compared with that obtained using the plasma technique under the same energy injection levels (typically 1015-18 atoms s-1 by injection of several hundreds of watts).16,24,25 In summary, a dominant AO emission from a solid oxide electrolyte into a vacuum was demonstrated for the very first time. The appearance potential spectroscopy can separate the AO directly emitted from the high-temperature surface of the zirconia and the MO formed by the recombination of AO. The AO emission is characterized by a higher intensity as compared with those of electron and O- ion emissions by at least a few orders of magnitude, significantly different activation energies from the O- ion, and no dependence on the extraction electric field. The dependence on the inner oxygen pressure supports the fact that the origin of AO is certainly an oxygen species diffusing through the zirconia and two possible origins for the AO emission were suggested. The employment of solid oxide electrolytes as an AO source provides unique merits over conventional plasma techniques including higher energy-efficiency, easier elimination of ionic species, and possible operation in wider pressure ranges. Acknowledgment. This work was supported by Grant-inAid for Industrial Technology Research (No. 05A25513a) from NEDO and Grants-in-Aid for Elements Strategy Project from MEXT, Japanese government. Supporting Information Available: S1. Experimental setup, S2. in situ measurement of atomic oxygen flux, S3. effect of dc bias, S4. oxidation of silver foil, and Figures S1-S5. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Zhu, J.; van Ommen, J. G.; Bouwmeester, H. J. M.; Lefferts, L. J. Catal. 2005, 233, 434. (2) Deluga, G. A.; Salge, J. R.; Schmidt, L. D.; Verykios, X. E. Science 2004, 303, 993. (3) Ozawa, M.; Kimura, M.; Isogai, A. J. Alloys Comp. 1993, 193, 73. (4) Gland, J. L.; Sexton, B. A.; Fisher, G. B. Surf. Sci. 1980, 95, 587. (5) (a) Davidson, M. R.; Hoflund, G. B.; Outlaw, R. A Surf. Sci. 1993, 281, 111. (b) Hoflund, G. B.; Weaver, J. F. Meas. Sci. Technol. 1994, 5, 201. (6) Chen, H.; Aleksandrov, A.; Zha, S.; Liu, M.; Orlando, T. M. J. Phys. Chem. B 2006, 110, 10779. (7) Nishioka, M.; Torimoto, Y.; Kashiwagi, H.; Li, Q.; Sadakata, M. J. Catal. 2003, 215, 1. (8) Fujiwara, Y.; Kaimai, A.; Hong, J.-O.; Yashiro, K.; Nigara, Y.; Kawada, T.; Mizusaki, J. J. Electrochem. Soc. 2003, 150, E117.
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J. Phys. Chem. C, Vol. 113, No. 22, 2009 9439 (19) Panov, G. I.; Uriarte, A. K.; Rodkin, M. A.; Sobolev, V. I. Catal. Today 1998, 41, 365. (20) Nowotny, J.; Bak, T.; Nowotny, M. K.; Sorrell, C. C. AdV. Appl. Ceram. 2005, 104, 147. (21) Merkle, R.; Maier, J. Phys. Chem. Chem. Phys. 2002, 4, 4140. (22) Busch, B. W.; Schulte, W. H.; Garfunkel, E.; Gustafsson, T. Phys. ReV. B. 2000, 62, R13290. (23) Foster, A. S.; Sulimov, V. B.; Lopez Gejo, F.; Shluger, A. L.; Nieminen, R. M. Phys. ReV. B 2001, 64, 224108. (24) Locquet, J. P.; Ma¨chler, E. J. Vac. Sci. Technol., A 1992, 10, 3100. (25) Ingle, N. J. C.; Hammond, R. H.; Beasley, M. R.; Blank, D. H. A. Appl. Phys. Lett. 1999, 75, 4162. (26) (a) Kazor, A.; Boyd, I. W. Appl. Phys. Lett. 1993, 63, 2517. (b) Nishiguchi, T.; Nonaka, H.; Ichimura, S.; Morikawa, Y.; Kekura, M.; Miyamoto, M. Appl. Phys. Lett. 2002, 81, 2190. (27) Wisotzki, E.; Balogh, A. G.; Hahn, B.; Wolan, J. T.; Hoflund, G. B. J. Vac. Sci. Technol., A 1999, 17, 14.
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