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Effects of humidified atmosphere on oxygen transport properties in La2Mo2O9
Solid State Ionics 192 (2011) 444–447
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Effects of humidified atmosphere on oxygen transport properties in La2Mo2O9 Jingjing Liu, Richard J. Chater, Stephen J. Skinner ⁎ Department of Materials, Imperial College London, Prince Consort Road, London SW7 2BP, UK
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 1 September 2009 Accepted 22 March 2010 Available online 24 April 2010
The effects of humidified atmosphere on oxygen surface exchange and diffusion in La2Mo2O9 have been investigated. After annealing samples in D2O vapour, the depth and line scan profiles of the OD- species showed the incorporation of hydroxyl groups on the surface and of diffusion into the bulk. The hydroxyl diffusion process appears to be different from that of oxygen diffusion, and might indicate the existence of ambipolar diffusion of oxide ions and hydroxyl species in La2Mo2O9. © 2010 Elsevier B.V. All rights reserved.
Keywords: La2Mo2O9 Oxygen transport Water vapour Oxygen isotope exchange SIMS
1. Introduction For both pure ionic and mixed conductors, oxygen transport properties are very important as they affect the electrochemical performance of the materials. To investigate oxygen transport in oxides, oxygen isotopic exchange followed by a diffusion measurement with secondary ion mass spectrometry (SIMS) has been applied as an effective method to obtain both the oxygen diffusion (D*) and surface exchange (k) coefficients [1,2]. However, due to the low (negligible) electronic conductivity in pure ionic conductors, oxygen surface exchange can be limited, which leads to difficulties with the oxygen diffusion measurement [3]. A common solution is to perform the isotopic exchange in water vapour because it is suggested that water adsorbed on the surface plays a significant part in the dissociation and reduction of oxygen molecules during the surface exchange process [4]. Sakai et al. [5] suggested that the oxygen isotope exchange reactions in wet atmospheres could be formulated as: 18
18
H2 OðgÞ →H2 Oad 18
H2 Oad + ••
VO + 16
18
16
ð1Þ
•• OO →VO +
0
OHad +
16
0
18
0
OHad + 16
16
OHad →H2 Oad +
0
OHad
ð2Þ
OO
ð3Þ
18
16
H2 Oad →H2 OðgÞ
⁎ Corresponding author. Tel.: + 44 20 7594 6782; fax: + 44 20 7594 6757. E-mail address: [email protected] (S.J. Skinner). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.03.021
ð4Þ
It should be noted that in these surface exchange reactions oxygen reduction is not included unlike the corresponding reactions in a dry oxygen atmosphere where the reduction of oxygen molecules requires electron availability on the electrolyte surface. Therefore, in a humidified atmosphere, the surface exchange is not limited by the availability of electrons and the fast reaction between water molecules and oxygen vacancies in the electrolyte enhances oxygen surface exchange drastically compared with the exchange in a dry atmosphere [4,6]. However, the chemical state of adsorbed water molecules and the reactions in the vicinity of the electrolyte seem to be different in different materials. In proton conducting ceramics such as BaCeO3, it is well known that the water molecules react with oxygen vacancies in oxides: ••
0
H2 OðgÞ + VO ↔OO + 2Hi
ð5Þ
In this case the interstitial protons migrate and act as charge carriers instead of electronic holes [7]. With an yttrium stabilized zirconia (YSZ) oxide ion conductor, it is suggested that the proton combines with lattice oxygen in the electrolyte to form a hydroxyl group (16OHad′ in Eq. (2)) which exists stably in the oxide [8]. In this material, besides the surface exchange, the humidified atmosphere has been discovered to have an effect on the oxygen diffusion coefficient. Sakai et al. [4] have found that the oxygen diffusion coefficients obtained with a wet exchange were lower than those measured in a dry atmosphere. However, the effects of water vapour on oxygen surface exchange and diffusion for gadolinium substituted ceria (CGO) were found not to be as dramatic as they were in YSZ [8]. Hence, what has been discussed above indicates that the reactions in
J. Liu et al. / Solid State Ionics 192 (2011) 444–447
445
the vicinity of the electrolyte in a humidified atmosphere are complicated and dependent on the individual material. La2Mo2O9, as a new fast ionic conductor [9], exhibits high ionic conductivity comparable to that of commercial YSZ at temperatures of up to 800 °C. To date, there have been few studies carried out on oxygen isotope exchange and diffusion in this material. The only direct study of oxide ion diffusion [10] was conducted in a wet atmosphere and did not investigate the effect of water vapour on the surface exchange and diffusion in La2Mo2O9. In our previous work [11], the oxygen isotope exchange and diffusion coefficients were determined in a humidified atmosphere. As discussed previously, concerning the surface interaction between water molecules and the electrolyte material, there is a possibility of proton incorporation or introduction of hydroxyl groups at the surface. In our current work, further studies were undertaken to investigate the species involved in the surface reaction and whether it affects oxygen diffusion in the material. 2. Experimental La2Mo2O9 powder was prepared by conventional solid state synthesis using La2O3 (dried at 1000 °C, 6 h) and MoO3 starting oxides at a synthesis temperature of 900 °C for 24 h. The pellet processing and sample preparation for the oxygen isotope exchange and diffusion measurements have been described elsewhere [11]. All samples were checked for phase purity using a Philips PW 1700 series X-ray diffractometer with Cu Kα radiation. To study the incorporation of proton or hydroxyl species, a sample was pre-annealed at 450 °C in 60 mbar H2O vapour with 200 mbar O2 as the carrying gas for 150 min, followed by exchange in 60 mbar D2O vapour with 200 mbar O2 gas at 450 °C for 15 min. Another two samples were annealed in 60 mbar D2O vapour with 200 mbar O2 gas at 450 °C and 800 °C respectively for 15 min without pre-annealing. After annealing, the samples were prepared for the subsequent SIMS analysis [6] in which depth profiles were measured to obtain the surface concentration of selected oxygen and hydrogen containing negative ion species as shown in Table 1. A line scan method was also used to measure the diffusion profile of each species. The SIMS instrument used in this work was an Atomika 6500 with argon primary ion beam. The primary ion beam energy was 5 KeV with a sample current of about 5 nA, giving a beam size of approximately 8 μm.
Fig. 1. Oxygen diffusion coefficients of La2Mo2O9 obtained from dry and wet exchange [11]. 95% confidence bands of linear fitting are displayed for both data sets.
levels which were obtained from SIMS in a sample wet exchanged at 450 °C for 30 min. The mass at m/e = 17 is mainly assigned to 16OH− species and the mass at m/e = 18 is assigned to 18O− species [12]. Apart from 18O− diffusion, the profile of the signal at m/e = 17 indicates the possible diffusion of 16OH− species. However, the impact from adsorbed water molecules on the sample surface and the residual gas in the SIMS chamber cannot be excluded. In order to eliminate the influence of water from the environment, we adopted D2O water vapour in the exchange atmosphere to determine any incorporation and diffusion of the hydroxyl species. The problem we encountered with the measurement was mass interference which means the occurrence of two or more secondary ions with identical mass/charge ratio. De Souza et al. have investigated the possibility of the existence of OHn species and identified the ions in the m/e range 16–19 [12]. According to their studies, it is less likely to have OHn (n ≥ 2) species and therefore 19Fˉ is the possible alternative species at m/e = 19 and 16OHˉ at m/e = 17. In the case of applying isotopic hydrogen (deuterium) instead of isotopic oxygen (18O) in the exchange, the possible mass interferences for negative ions in the m/e range of 16–20 are listed in Table 1.
3. Results and discussion In our previous studies [11], by comparing the oxygen diffusion coefficients obtained in dry and wet atmospheres (Fig. 1), we have observed that D* values obtained from the wet exchange were slightly lower than those from the dry exchange, but considering the 95% confidence band in the linear fitting, the difference is not significant. On the other hand, interestingly, a profile of ion species at m/e = 17 was observed in the sample which was wet exchanged (oxygen isotope exchange in H18 2 O) at different temperatures, indicating that there were further diffusing species and not only 18O−. Fig. 2 shows the profiles of ion species at m/e = 17 and m/e = 18 respectively with their background Table 1 Possible mass interferences for negative ions in the sample annealed in D2O. Mass/charge
Possible species
16 17 18 19 20
16
O (99.76%) O (0.04%) 18 O (0.20%) F (100%) 17
16
OH (99.76%*99.985%) OH (0.04%*99.985%) OH (0.20%*99.985%)
17 18
OD (99.76%*0.015%) OD (0.04%*0.015%) OD (0.20%*0.015%)
17 18
* The natural abundance of each species is displayed in the bracket (99.985% is the isotopic fraction of 1H and 0.015% is the isotopic fraction of 2H (D)).
Fig. 2. The depth profiles of species at m/e = 17 and 18 respectively in a sample which was exchanged in H18 2 O vapour at 450 °C for 30 min highlighting the possible OH diffusion profile.
446
J. Liu et al. / Solid State Ionics 192 (2011) 444–447
The first sample was pre-annealed in H2O followed by exchange in D2O for 15 min. The aim of the pre-annealing is to equilibrate the sample [13] if there is a flux between OH− and OD− species during the exchange. The relative concentration of the species at m/e = 18 compared to the species at m/e = 16 (Im/e= 18 / (Im/e= 18 + Im/e= 16) where I = intensity of the mass signal) in the sample is shown in Fig. 3(a). According to Table 1, m/e = 16 is assigned to 16O−, while m/ e = 18 can be assigned to 18O−, 17OH− and OD−. The contribution from 17 OH− is negligible because of the low natural abundance of this oxygen isotope, 17O (0.04%). It is apparent that the relative concentration of the species at m/e = 18 is much higher than the natural abundance of 18O (0.20%), which implies that the high intensity is due to the existence of the OD- species. To exclude the effect of surface incorporation and adsorption, a line scan was carried out on the sectioned surface of the sample and the ratio of the signal at m/e = 18 to the 16O counts as a function of diffusion length is shown in Fig. 3(b). Note that the initial concentration of the line scan profile is lower than the value calculated from the depth profile of the surface, because in a humidified atmosphere, chemisorption and physisorption of water molecules can result in a thin layer of water adsorbed on the oxide [14]. The isotope concentration on the sample surface was likely affected by the water adsorption layer. The profile is observed to decrease before reaching a plateau at a concentration of 0.002 which is the 18O natural abundance (background). This indicates that there is a diffusion profile of the ODspecies of about 30 μm depth, but the intensity is very low which means
there is only a small degree of incorporation of hydroxyl groups. Note we did not conduct a profile at m/e = 2 because of the mass resolution limit of the instrument and the significant influence of the hydrogen signal at m/e = 1. Therefore, the question of whether protons exist stably as interstitials in the structure is not yet clear. As discussed above, in a humidified atmosphere, the hydroxyl groups generated during the surface reactions can be incorporated into the bulk and diffuse through the material. If the hydroxyl groups tend to occupy oxygen vacancies, the pre-annealing in H2O may have caused a decrease in the concentration of oxygen vacancies which might affect further incorporation of OD− groups when the atmosphere is changed to D2O. Therefore, a further two samples were annealed in D2O vapour without pre-annealing in H2O atmosphere. Here it should be noted that it is likely that there will be a chemical potential gradient which affects the surface exchange and diffusion processes. These experiments were carried out in D2O at 450 °C and 800 °C respectively, to investigate if there is any difference in the hydroxyl incorporation between the low temperature α-La2Mo2O9 phase and the more conductive high temperature phase, β-La2Mo2O9. The depth profiles of the two samples are shown in Fig. 4(a). The relative concentration of the mass at m/e = 18 compared to 16O is higher than the 18O natural abundance, which suggests that there was surface incorporation of the OD− species. The surface intensity of ODin the sample annealed at 800 °C is lower than that in the sample
Fig. 3. (a) The relative concentration of the species at m/e = 18 compared to that at m/ e = 16 in the depth profile of the sample which was pre-annealed in H2O followed by exchange in D2O for 15 min. (b) Line scan profile of the signal at m/e = 18 compared to 16 O as a function of depth into the sample.
Fig. 4. (a) The relative concentrations of the species of m/e = 18 compared to that of m/ e = 16 in the depth profiles of the samples which were annealed in D2O at 450 °C and 800 °C for 15 min respectively. (b) Line scan profile of the signal at m/e = 18 compared to 16O as a function of depth in the two samples. A trend line is fitted through each data set to reflect the diffusion profile.
J. Liu et al. / Solid State Ionics 192 (2011) 444–447
exchanged at 450 °C. One explanation is that the fast oxygen diffusion minimized the build-up of oxygen ions (hydroxyl groups) near the surface. It could also be due to the difference in the mobility of hydroxyl species in α- and β-La2Mo2O9. Further investigation is required to interpret this phenomenon. The line scan results for these two samples are displayed in Fig. 4(b). The diffusion length of the sample annealed at 450 °C was found to be similar to the diffusion length in the sample with H2O pre-annealing. It seems that the pre-annealing step did not have any significant influence on the hydroxyl incorporation, which may imply that the effect of chemical incorporation on the surface is not significant, or that the mechanism of hydroxyl species incorporation is different from that of oxygen isotope exchange. The diffusion length did not vary significantly at the two temperatures, indicating that hydroxyl incorporation is not significantly affected by temperature, which is substantially different from the dramatic increase of the oxygen diffusion coefficient when the phase changes to β-La2Mo2O9 [10], which might also suggest that the oxygen diffusion and the hydroxyl diffusion are two separate processes. This may suggest ambipolar diffusion and hence indicate possible steam permeation as suggested in the proton conductors reported by Coors et al. [15,16]. However, the diffusion profiles of the two samples are slightly different, suggesting that there may be a difference in hydroxyl mobility in the two phases of La2Mo2O9. 4. Conclusions Differences in oxygen diffusion coefficients in La2Mo2O9 were observed when the isotope exchange was carried out in both dry and wet atmospheres in earlier studies. Although oxygen diffusion coefficients were not significantly different from the values in dry exchange, a possibility of hydroxyl incorporation was suggested. In this work, we annealed samples in D2O vapour to study the incorporation and diffusion of hydroxyl species in La2Mo2O9. It is
447
evident from the results that there is significant incorporation of hydroxyl groups in the vicinity of the electrolyte surface and a short diffusion into the bulk in both low temperature and high temperature phases. Further work is under way to investigate if the incorporation is dependent on temperature or annealing time. Moreover, more work is required to investigate if the lattice oxygen ions are involved in forming hydroxyl groups with the aim of determining whether the hydroxyl groups migrate via proton hopping among oxygen ions in the lattice structure or via oxygen vacancies as a whole. Acknowledgements The authors would like to thank the Lee family for providing funding for a studentship (JL) and would further like to thank Prof J. Kilner for useful discussions. References [1] J.A. Kilner, B.C.H. Steele, Solid State Ionics 12 (1984) 89. [2] R.J. Chater, S. Carter, J.A. Kilner, B.C.H. Steele, Solid State Ionics 53–56 (1992) 859. [3] B.A. Boukamp, B.A. Van Hassel, I.C. Vinke, K.J. De Vries, A.J. Burggruf, Electrochim. Acta 38 (1993) 1817. [4] N. Sakai, K. Yamaji, T. Horita, J. Electrochem. Soc. 150 (2003) A689. [5] N. Sakai, K. Yamaji, T. Horita, Y.P. Xiong, H. Kishimoto, M.E. Brito, H. Yokokawa, Solid State Ionics 176 (2005) 2327. [6] P.S. Manning, J.D. Sirman, J.A. Kilner, Solid State Ionics 93 (1997) 125. [7] Y. Larring, T. Norby, Solid State Ionics 77 (1995) 147. [8] N. Sakai, K. Yamaji, T. Horita, H. Kishimoto, Y.P. Xiong, H. Yokokawa, Phys. Chem. Chem. Phys. 5 (2003) 2253. [9] P. Lacorre, F. Goutenoire, O. Bohnke, Nature 404 (2000) 856. [10] S. Georges, S.J. Skinner, P. Lacorre, M.C. Steil, Dalton Trans. 19 (2004) 3101. [11] J. Liu, PhD Thesis, Imperial College London, London, 2010. [12] R.A. De Souza, J.A. Kilner, B.C.H. Steele, Solid State Ionics 77 (1995) 180. [13] R.A. De Souza, R.J. Chater, Solid State Ionics 176 (2005) 1915. [14] S. Raz, K. Sasaki, J. Maier, I. Riess, Solid State Ionics 143 (2001) 181. [15] W. Grover Coors, Solid State Ionics 178 (2007) 481. [16] T. Schober, W. Grover Coors, Solid State Ionics 176 (2005) 357.
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Effects of humidified atmosphere on oxygen transport properties in La2Mo2O9 Jingjing Liu, Richard J. Chater, Stephen J. Skinner ⁎ Department of Materials, Imperial College London, Prince Consort Road, London SW7 2BP, UK
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 1 September 2009 Accepted 22 March 2010 Available online 24 April 2010
The effects of humidified atmosphere on oxygen surface exchange and diffusion in La2Mo2O9 have been investigated. After annealing samples in D2O vapour, the depth and line scan profiles of the OD- species showed the incorporation of hydroxyl groups on the surface and of diffusion into the bulk. The hydroxyl diffusion process appears to be different from that of oxygen diffusion, and might indicate the existence of ambipolar diffusion of oxide ions and hydroxyl species in La2Mo2O9. © 2010 Elsevier B.V. All rights reserved.
Keywords: La2Mo2O9 Oxygen transport Water vapour Oxygen isotope exchange SIMS
1. Introduction For both pure ionic and mixed conductors, oxygen transport properties are very important as they affect the electrochemical performance of the materials. To investigate oxygen transport in oxides, oxygen isotopic exchange followed by a diffusion measurement with secondary ion mass spectrometry (SIMS) has been applied as an effective method to obtain both the oxygen diffusion (D*) and surface exchange (k) coefficients [1,2]. However, due to the low (negligible) electronic conductivity in pure ionic conductors, oxygen surface exchange can be limited, which leads to difficulties with the oxygen diffusion measurement [3]. A common solution is to perform the isotopic exchange in water vapour because it is suggested that water adsorbed on the surface plays a significant part in the dissociation and reduction of oxygen molecules during the surface exchange process [4]. Sakai et al. [5] suggested that the oxygen isotope exchange reactions in wet atmospheres could be formulated as: 18
18
H2 OðgÞ →H2 Oad 18
H2 Oad + ••
VO + 16
18
16
ð1Þ
•• OO →VO +
0
OHad +
16
0
18
0
OHad + 16
16
OHad →H2 Oad +
0
OHad
ð2Þ
OO
ð3Þ
18
16
H2 Oad →H2 OðgÞ
⁎ Corresponding author. Tel.: + 44 20 7594 6782; fax: + 44 20 7594 6757. E-mail address: [email protected] (S.J. Skinner). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.03.021
ð4Þ
It should be noted that in these surface exchange reactions oxygen reduction is not included unlike the corresponding reactions in a dry oxygen atmosphere where the reduction of oxygen molecules requires electron availability on the electrolyte surface. Therefore, in a humidified atmosphere, the surface exchange is not limited by the availability of electrons and the fast reaction between water molecules and oxygen vacancies in the electrolyte enhances oxygen surface exchange drastically compared with the exchange in a dry atmosphere [4,6]. However, the chemical state of adsorbed water molecules and the reactions in the vicinity of the electrolyte seem to be different in different materials. In proton conducting ceramics such as BaCeO3, it is well known that the water molecules react with oxygen vacancies in oxides: ••
0
H2 OðgÞ + VO ↔OO + 2Hi
ð5Þ
In this case the interstitial protons migrate and act as charge carriers instead of electronic holes [7]. With an yttrium stabilized zirconia (YSZ) oxide ion conductor, it is suggested that the proton combines with lattice oxygen in the electrolyte to form a hydroxyl group (16OHad′ in Eq. (2)) which exists stably in the oxide [8]. In this material, besides the surface exchange, the humidified atmosphere has been discovered to have an effect on the oxygen diffusion coefficient. Sakai et al. [4] have found that the oxygen diffusion coefficients obtained with a wet exchange were lower than those measured in a dry atmosphere. However, the effects of water vapour on oxygen surface exchange and diffusion for gadolinium substituted ceria (CGO) were found not to be as dramatic as they were in YSZ [8]. Hence, what has been discussed above indicates that the reactions in
J. Liu et al. / Solid State Ionics 192 (2011) 444–447
445
the vicinity of the electrolyte in a humidified atmosphere are complicated and dependent on the individual material. La2Mo2O9, as a new fast ionic conductor [9], exhibits high ionic conductivity comparable to that of commercial YSZ at temperatures of up to 800 °C. To date, there have been few studies carried out on oxygen isotope exchange and diffusion in this material. The only direct study of oxide ion diffusion [10] was conducted in a wet atmosphere and did not investigate the effect of water vapour on the surface exchange and diffusion in La2Mo2O9. In our previous work [11], the oxygen isotope exchange and diffusion coefficients were determined in a humidified atmosphere. As discussed previously, concerning the surface interaction between water molecules and the electrolyte material, there is a possibility of proton incorporation or introduction of hydroxyl groups at the surface. In our current work, further studies were undertaken to investigate the species involved in the surface reaction and whether it affects oxygen diffusion in the material. 2. Experimental La2Mo2O9 powder was prepared by conventional solid state synthesis using La2O3 (dried at 1000 °C, 6 h) and MoO3 starting oxides at a synthesis temperature of 900 °C for 24 h. The pellet processing and sample preparation for the oxygen isotope exchange and diffusion measurements have been described elsewhere [11]. All samples were checked for phase purity using a Philips PW 1700 series X-ray diffractometer with Cu Kα radiation. To study the incorporation of proton or hydroxyl species, a sample was pre-annealed at 450 °C in 60 mbar H2O vapour with 200 mbar O2 as the carrying gas for 150 min, followed by exchange in 60 mbar D2O vapour with 200 mbar O2 gas at 450 °C for 15 min. Another two samples were annealed in 60 mbar D2O vapour with 200 mbar O2 gas at 450 °C and 800 °C respectively for 15 min without pre-annealing. After annealing, the samples were prepared for the subsequent SIMS analysis [6] in which depth profiles were measured to obtain the surface concentration of selected oxygen and hydrogen containing negative ion species as shown in Table 1. A line scan method was also used to measure the diffusion profile of each species. The SIMS instrument used in this work was an Atomika 6500 with argon primary ion beam. The primary ion beam energy was 5 KeV with a sample current of about 5 nA, giving a beam size of approximately 8 μm.
Fig. 1. Oxygen diffusion coefficients of La2Mo2O9 obtained from dry and wet exchange [11]. 95% confidence bands of linear fitting are displayed for both data sets.
levels which were obtained from SIMS in a sample wet exchanged at 450 °C for 30 min. The mass at m/e = 17 is mainly assigned to 16OH− species and the mass at m/e = 18 is assigned to 18O− species [12]. Apart from 18O− diffusion, the profile of the signal at m/e = 17 indicates the possible diffusion of 16OH− species. However, the impact from adsorbed water molecules on the sample surface and the residual gas in the SIMS chamber cannot be excluded. In order to eliminate the influence of water from the environment, we adopted D2O water vapour in the exchange atmosphere to determine any incorporation and diffusion of the hydroxyl species. The problem we encountered with the measurement was mass interference which means the occurrence of two or more secondary ions with identical mass/charge ratio. De Souza et al. have investigated the possibility of the existence of OHn species and identified the ions in the m/e range 16–19 [12]. According to their studies, it is less likely to have OHn (n ≥ 2) species and therefore 19Fˉ is the possible alternative species at m/e = 19 and 16OHˉ at m/e = 17. In the case of applying isotopic hydrogen (deuterium) instead of isotopic oxygen (18O) in the exchange, the possible mass interferences for negative ions in the m/e range of 16–20 are listed in Table 1.
3. Results and discussion In our previous studies [11], by comparing the oxygen diffusion coefficients obtained in dry and wet atmospheres (Fig. 1), we have observed that D* values obtained from the wet exchange were slightly lower than those from the dry exchange, but considering the 95% confidence band in the linear fitting, the difference is not significant. On the other hand, interestingly, a profile of ion species at m/e = 17 was observed in the sample which was wet exchanged (oxygen isotope exchange in H18 2 O) at different temperatures, indicating that there were further diffusing species and not only 18O−. Fig. 2 shows the profiles of ion species at m/e = 17 and m/e = 18 respectively with their background Table 1 Possible mass interferences for negative ions in the sample annealed in D2O. Mass/charge
Possible species
16 17 18 19 20
16
O (99.76%) O (0.04%) 18 O (0.20%) F (100%) 17
16
OH (99.76%*99.985%) OH (0.04%*99.985%) OH (0.20%*99.985%)
17 18
OD (99.76%*0.015%) OD (0.04%*0.015%) OD (0.20%*0.015%)
17 18
* The natural abundance of each species is displayed in the bracket (99.985% is the isotopic fraction of 1H and 0.015% is the isotopic fraction of 2H (D)).
Fig. 2. The depth profiles of species at m/e = 17 and 18 respectively in a sample which was exchanged in H18 2 O vapour at 450 °C for 30 min highlighting the possible OH diffusion profile.
446
J. Liu et al. / Solid State Ionics 192 (2011) 444–447
The first sample was pre-annealed in H2O followed by exchange in D2O for 15 min. The aim of the pre-annealing is to equilibrate the sample [13] if there is a flux between OH− and OD− species during the exchange. The relative concentration of the species at m/e = 18 compared to the species at m/e = 16 (Im/e= 18 / (Im/e= 18 + Im/e= 16) where I = intensity of the mass signal) in the sample is shown in Fig. 3(a). According to Table 1, m/e = 16 is assigned to 16O−, while m/ e = 18 can be assigned to 18O−, 17OH− and OD−. The contribution from 17 OH− is negligible because of the low natural abundance of this oxygen isotope, 17O (0.04%). It is apparent that the relative concentration of the species at m/e = 18 is much higher than the natural abundance of 18O (0.20%), which implies that the high intensity is due to the existence of the OD- species. To exclude the effect of surface incorporation and adsorption, a line scan was carried out on the sectioned surface of the sample and the ratio of the signal at m/e = 18 to the 16O counts as a function of diffusion length is shown in Fig. 3(b). Note that the initial concentration of the line scan profile is lower than the value calculated from the depth profile of the surface, because in a humidified atmosphere, chemisorption and physisorption of water molecules can result in a thin layer of water adsorbed on the oxide [14]. The isotope concentration on the sample surface was likely affected by the water adsorption layer. The profile is observed to decrease before reaching a plateau at a concentration of 0.002 which is the 18O natural abundance (background). This indicates that there is a diffusion profile of the ODspecies of about 30 μm depth, but the intensity is very low which means
there is only a small degree of incorporation of hydroxyl groups. Note we did not conduct a profile at m/e = 2 because of the mass resolution limit of the instrument and the significant influence of the hydrogen signal at m/e = 1. Therefore, the question of whether protons exist stably as interstitials in the structure is not yet clear. As discussed above, in a humidified atmosphere, the hydroxyl groups generated during the surface reactions can be incorporated into the bulk and diffuse through the material. If the hydroxyl groups tend to occupy oxygen vacancies, the pre-annealing in H2O may have caused a decrease in the concentration of oxygen vacancies which might affect further incorporation of OD− groups when the atmosphere is changed to D2O. Therefore, a further two samples were annealed in D2O vapour without pre-annealing in H2O atmosphere. Here it should be noted that it is likely that there will be a chemical potential gradient which affects the surface exchange and diffusion processes. These experiments were carried out in D2O at 450 °C and 800 °C respectively, to investigate if there is any difference in the hydroxyl incorporation between the low temperature α-La2Mo2O9 phase and the more conductive high temperature phase, β-La2Mo2O9. The depth profiles of the two samples are shown in Fig. 4(a). The relative concentration of the mass at m/e = 18 compared to 16O is higher than the 18O natural abundance, which suggests that there was surface incorporation of the OD− species. The surface intensity of ODin the sample annealed at 800 °C is lower than that in the sample
Fig. 3. (a) The relative concentration of the species at m/e = 18 compared to that at m/ e = 16 in the depth profile of the sample which was pre-annealed in H2O followed by exchange in D2O for 15 min. (b) Line scan profile of the signal at m/e = 18 compared to 16 O as a function of depth into the sample.
Fig. 4. (a) The relative concentrations of the species of m/e = 18 compared to that of m/ e = 16 in the depth profiles of the samples which were annealed in D2O at 450 °C and 800 °C for 15 min respectively. (b) Line scan profile of the signal at m/e = 18 compared to 16O as a function of depth in the two samples. A trend line is fitted through each data set to reflect the diffusion profile.
J. Liu et al. / Solid State Ionics 192 (2011) 444–447
exchanged at 450 °C. One explanation is that the fast oxygen diffusion minimized the build-up of oxygen ions (hydroxyl groups) near the surface. It could also be due to the difference in the mobility of hydroxyl species in α- and β-La2Mo2O9. Further investigation is required to interpret this phenomenon. The line scan results for these two samples are displayed in Fig. 4(b). The diffusion length of the sample annealed at 450 °C was found to be similar to the diffusion length in the sample with H2O pre-annealing. It seems that the pre-annealing step did not have any significant influence on the hydroxyl incorporation, which may imply that the effect of chemical incorporation on the surface is not significant, or that the mechanism of hydroxyl species incorporation is different from that of oxygen isotope exchange. The diffusion length did not vary significantly at the two temperatures, indicating that hydroxyl incorporation is not significantly affected by temperature, which is substantially different from the dramatic increase of the oxygen diffusion coefficient when the phase changes to β-La2Mo2O9 [10], which might also suggest that the oxygen diffusion and the hydroxyl diffusion are two separate processes. This may suggest ambipolar diffusion and hence indicate possible steam permeation as suggested in the proton conductors reported by Coors et al. [15,16]. However, the diffusion profiles of the two samples are slightly different, suggesting that there may be a difference in hydroxyl mobility in the two phases of La2Mo2O9. 4. Conclusions Differences in oxygen diffusion coefficients in La2Mo2O9 were observed when the isotope exchange was carried out in both dry and wet atmospheres in earlier studies. Although oxygen diffusion coefficients were not significantly different from the values in dry exchange, a possibility of hydroxyl incorporation was suggested. In this work, we annealed samples in D2O vapour to study the incorporation and diffusion of hydroxyl species in La2Mo2O9. It is
447
evident from the results that there is significant incorporation of hydroxyl groups in the vicinity of the electrolyte surface and a short diffusion into the bulk in both low temperature and high temperature phases. Further work is under way to investigate if the incorporation is dependent on temperature or annealing time. Moreover, more work is required to investigate if the lattice oxygen ions are involved in forming hydroxyl groups with the aim of determining whether the hydroxyl groups migrate via proton hopping among oxygen ions in the lattice structure or via oxygen vacancies as a whole. Acknowledgements The authors would like to thank the Lee family for providing funding for a studentship (JL) and would further like to thank Prof J. Kilner for useful discussions. References [1] J.A. Kilner, B.C.H. Steele, Solid State Ionics 12 (1984) 89. [2] R.J. Chater, S. Carter, J.A. Kilner, B.C.H. Steele, Solid State Ionics 53–56 (1992) 859. [3] B.A. Boukamp, B.A. Van Hassel, I.C. Vinke, K.J. De Vries, A.J. Burggruf, Electrochim. Acta 38 (1993) 1817. [4] N. Sakai, K. Yamaji, T. Horita, J. Electrochem. Soc. 150 (2003) A689. [5] N. Sakai, K. Yamaji, T. Horita, Y.P. Xiong, H. Kishimoto, M.E. Brito, H. Yokokawa, Solid State Ionics 176 (2005) 2327. [6] P.S. Manning, J.D. Sirman, J.A. Kilner, Solid State Ionics 93 (1997) 125. [7] Y. Larring, T. Norby, Solid State Ionics 77 (1995) 147. [8] N. Sakai, K. Yamaji, T. Horita, H. Kishimoto, Y.P. Xiong, H. Yokokawa, Phys. Chem. Chem. Phys. 5 (2003) 2253. [9] P. Lacorre, F. Goutenoire, O. Bohnke, Nature 404 (2000) 856. [10] S. Georges, S.J. Skinner, P. Lacorre, M.C. Steil, Dalton Trans. 19 (2004) 3101. [11] J. Liu, PhD Thesis, Imperial College London, London, 2010. [12] R.A. De Souza, J.A. Kilner, B.C.H. Steele, Solid State Ionics 77 (1995) 180. [13] R.A. De Souza, R.J. Chater, Solid State Ionics 176 (2005) 1915. [14] S. Raz, K. Sasaki, J. Maier, I. Riess, Solid State Ionics 143 (2001) 181. [15] W. Grover Coors, Solid State Ionics 178 (2007) 481. [16] T. Schober, W. Grover Coors, Solid State Ionics 176 (2005) 357.