- Email: [email protected]
Proton-conducting BaO-P2O5 and BaO-La2O3-Al2O3-P2O5 glass thin films
Solid State Ionics 345 (2020) 115186
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Proton-conducting BaO-P2O5 and BaO-La2O3-Al2O3-P2O5 glass thin films Suk Hee Lee, Sung Bum Park, Yong-il Park
⁎
T
School of Materials Science and Engineering, Kumoh National Institute of Technology, Gumi, Gyeongbuk, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: BaO-P2O5 BaO-La2O3-Al2O3-P2O5 Phosphate glass Thin film Proton conduction
BaO-P2O5 (BP) and BaO-La2O3-Al2O3-P2O5 (BLAP) glass thin films with a thickness of 80–160 nm, which have a high content of residual moisture and a high concentration of non-bridging oxygen (NBO) as a medium for proton movement, were fabricated through sol-gel spin coating using water as a solvent. All the fabricated glass thin films exhibited amorphous phase and glassy, defect-free surfaces. The thickness of the BP thin films decreased as the annealing temperature increased. The increase in the annealing temperature led to a decrease in NBO/BO ratio which is thought to affect proton mobility and an increase in mobile proton concentration. Therefore, it seems that high proton concentration by residual water appears to have a dominant effect on fast proton conduction. The calculated proton concentrations of the fabricated BP thin films were decreased from 55.9 mol/l to 45.5 mol/l as the annealing temperature increased from 300 °C to 400 °C. The conductivity of BP and BLAP thin films annealed at 400 °C were 2.52 × 10−6 S/cm and 1.32 × 10−5 S/cm, respectively, at 300 °C. BLAP thin film showed significantly improved conductivity compared to those of BP glass thin films due to its high proton concentration.
1. Introduction Proton-conducting phosphate glasses have a low glass transition temperature, low melting temperature, and high ion conductivity, and can be applied to various electrochemical devices as electrolytes of fuel cells, hydrogen separating membranes, and sensors [1–6]. Unlike silica glasses, phosphate glasses are reported to have protons with a high mobility [7–9]. The role of protons as a charge carrier in silica glasses has long been denied owing to the high bonding strength of OeH. Thus, research into the electrical conduction of glass has mostly been limited to alkaline ionic conduction and electronic conduction [10,11]. Typically, in oxide glass, a proton has a lower mobility than an alkali ion. Doremus et al. [12] reported that the mobility of Na+ in silica glass is 104 times higher than that of H+ possibly because strong OeH bonds exist in silica glasses. Hydrogen bonds, which are abundant in the glass, differ depending on the type of oxygen that is primarily bonded. The hydrogen bonding force is the weakest in the form of X-O-X (bridging oxygen: BO), while it becomes stronger in the form of X-O– (nonbridging oxygen: NBO). The intensity of a hydrogen bond depends on the form of the inner network of the bonding cation. The concentration of water that remains after the fabrication of a glass is 0.01–0.5 wt%. This remaining water results in the formation of an X-OH glass network, significantly affecting the properties of the glass. H. Namikawa et al. [13] identified the phenomenon of proton
⁎
conduction in a BaO-P2O5 (BP) glass. It was confirmed that protons become the main source of movement in the phosphate glass. The significant factors in determining the binary glass composition are the durability against water to obtain relative conductivity and the ease of changing the water content over a wide range. The dependency on these two factors was confirmed by a previous study on binary phosphate glasses [14]. The order of proton mobility in an alkali earth phosphate glass was reported to be arranged by the alkali component, that is, Ba > Sr > Ca > Mg [5]. Later, Y. Abe et al. reported a superionic conducting glass having a conductivity of 10−4 S/cm which maintain a high proton conductivity even at moderately hot temperatures of 200 °C–400 °C. [14,15] Among them, proton conductivity of 22BaO-2.5La2O3–0.5Al2O3-75P2O5 (BLAP) bulk glass, which was prepared by melt-quenching of a mixture of H3PO4 and metal carbonates at 700 °C for 30 min, was extremely high, about 10−2 S/cm at 200 °C due to residual water which exists as H2O at low temperatures and as OH group at temperatures above 100 °C [16]. The factors affecting the proton conduction of phosphate glass are the amount of residual water, proton concentration, and the amount of phosphorus inside the glass [17]. In the case of proton conduction without presence of water molecule, the conduction occurs by a hopping mechanism through OeH group. However, in glass containing water molecules, a proton quickly moves through the water molecules. That is, the water molecules act as a catalyst by lowering the activation
Corresponding author. E-mail address: [email protected] (Y.-i. Park).
https://doi.org/10.1016/j.ssi.2019.115186 Received 5 October 2019; Received in revised form 10 November 2019; Accepted 9 December 2019 0167-2738/ © 2019 Elsevier B.V. All rights reserved.
Solid State Ionics 345 (2020) 115186
S.H. Lee, et al.
Aldrich]. The BLAP thin films was fabricated through the processes shown in Fig. 1(b). Phosphorus pentoxide [P2O5, Sigma-Aldrich] was mixed in distilled water for 20 min until it dissolved and then combined with aluminum dihydrogen phosphate [AlH6O12P3, Alfa Aesar] before stirring for another 10 min. Subsequently, lanthanum nitrate hexahydrate and barium nitrate were added and mixed for 20 min. For the precursor solution, unlike the composition of the reported bulk BLAP glass of BaO:La2O3:Al2O3:P2O5 = 22:2.5:0.5:75 (in molar ratio) an excess amount of phosphorus was initially added to the precursor solution. The molar ratio of P/(Ba+La + Al) was fixed at 6.0 as the same as precursor solution for BP glass thin films, which is 1.88 times of P/ (Ba+La + Al) ratio of the reported BLAP bulk glass. The BLAP gel films were deposited by spin-coating the precursor solution on pretreated Ptcoated Si wafers for 30 s at 3000 rpm. Then the gel films were dried at 140 °C for 10 min and annealed at 400 °C for 10 min (from now on, BLAP 400). This drying-annealing process was repeated five times to produce glass thin films. In Table 1, phosphorus content, calculated proton concentration and log Ao values of BP and BLAP thin films are summarized. To observe the microstructure of the fabricated thin film, FE-SEM (FE-SEM, JEOL model JSM-6500F) was used and the thin film thickness was measured using post-deposition FE-SEM cross sectional images. To examine the chemical bonding structure and proton mobility, FT-IR (FT-IR, Vertex 80v) analysis was performed. XRD (SWXD, X-MAX/ 2000-PC, Rigaku) was used to check the crystal structure in the range of 2θ = 10–60° at the speed of 0.5°/min. To examine the electrical properties, a resistance measurement was carried out using an AC impedance analyzer (Solartron SI 1260, ULVAC KIKO Inc.), increasing temperature from 150 °C to 350 °C in though-plane direction. The measuring frequency range was 0.1–106 Hz, and the applied voltage was 10 mV. To analyze the components of the fabricated phosphate glass, an X-ray photoelectron spectrometer (XPS) (Thermo Fisher Scientific, Multilab-2000) was used.
energy for proton conduction. Thus, the conductivity becomes 3–4 times higher than that of the glass without water molecules [18]. Because protons in silica glass bind very strongly to oxygen, which limits their movement, resulting in lower proton conductivity. On the other hand, phosphate glass has a PO4 tetrahedral structure where the phosphorus cation and oxygen ion form a covalent bond depending on the composition. Since phosphorus cations tend to form structures of non-bridging oxygen (NBO) rather than tetrahedral structures, OeH bonds are weak, while hydrogen bonds are strong. As a result, both the proton mobility and the proton conductivity are increased [19]. That is, stronger hydrogen bonds between the proton and nearby NBO give rise to a weaker bonding force of the OH group, leading to a significant increase in proton mobility and an increase in proton conductivity. In this regard, it has been reported that when hydrolysis occurs in a P-O-P structure of a phosphate network, the number of P-OH groups increases, thereby increasing the concentration of mobile protons [20]. In addition, increasing the concentration of NBO forms strong hydrogen bonds, increasing the proton conductivity. Therefore, increased phosphate content results in high proton conductivity in the temperature range above 100 °C [21]. As a result, phosphate glass is expected to maintain a high proton conductivity even at intermediate temperature, i.e., 200–400 °C. Proton conductivity increases with phosphorus content, however, materials based on phosphate glass have some drawbacks. The degradation of chemical durability occurs due to the severe vaporization of phosphorus during annealing [20]. Since phosphorus easily volatilizes at high temperatures, causing a decrease in content during heat treatment and readily bonds with water, so the glass structure is very susceptible to breakage. Furthermore, the volatilization of phosphorus becomes more severe in the two-dimensional thin film form, which is preferred geometry as an electrolyte form in many electrochemical devices [22,21]. The effect of phosphorus volatilization on electrical properties has not been seriously reported for bulk phosphate glasses, but it can have a significant effect, especially for thin films with submicrometer thickness. In this study, BP glasses were fabricated in the form of sol-gel glass thin film which has very high applicability to electronic devices such as fuel cells and gas sensors, and the effects of phosphorus loss during annealing and their electrical properties were investigated. Furthermore, BLAP glass, which has been reported to have higher proton mobility than BP glass thin film was also fabricated and compared with the fabricated BP glass thin films. Water was used as a solvent to improve the proton conductivity by increasing the concentration of NBO in the phosphate glass.
3. Results and discussion 3.1. Microstructure Fig. 2 shows the surface and the cross-sectional FE-SEM images of BP and BLAP glass thin films. The glass thin films were all homogeneous and free of defects, and their thickness ranged from 90 to 160 nm. The difference in thickness between BP 300 and 400 is ascribable to an increase in additional density with increasing annealing temperature. XRD patterns of Fig. 3 show that all the prepared glass thin films have an amorphous phase. The increase in the intensity around 2θ = 10° is due to nanopores, which are typically observed in sol-gel-derived glasses [24].
2. Experimental Using barium nitrate (Ba(NO3)2) as a starting material, phosphoric acid was dissolved in distilled water to fabricate the precursor solution. As for composition of the precursor solution, an excess amount of phosphorus was initially added to the precursor solution. The P/Ba ratio was adjusted to 3.0 which is 1.5 times of the melt-quenched BaOP2O5 glass. After stirring barium nitrate (Ba(NO3)2) in distilled water for 20 min, the solution was mixed with a phosphoric acid solution and stirred again for 20 min. Then, spin coating was performed using the prepared solution on a Pt-coated Si-wafer substrate at 3000 rpm for 30 s to deposit a gel film. The gel film was dried at 140 °C for 10 min. In order to remove organic impurities and solvents that possibly remained in the thin film, heat- treatment was performed at 300 °C for 10 min and at 400 °C for 10 min. This process was repeated six times so that the total time taken for the annealing was 60 min at each 300 and 400 °C (from now on, BP 300 and 400, respectively.). Fig. 1(a) shows the process sequence for the BP glass thin films. To fabricate BLAP glass thin films, a clear precursor sol was synthesized using barium nitrate [Ba(NO3)2, Daejung], lanthanum nitrate hexahydrate [La(NO3)3·6H2O, Wako], aluminum dihydrogen phosphate [AlH6O12P3, Alfa Aesar], and phosphorus pentoxide [P2O5, Sigma-
3.2. Moisture and proton mobility Scholze et al. [25] studied the form and bonding state of the H2O in oxide glass and concluded that moisture within molecules does not exist in glass made by the typical melt-quenching process. Since protons in conventional silica glass bond to oxygen very strongly, hydrogen bonds are not formed, yielding low proton conductivity. However, in phosphate glass, the moisture remains in the form of OH (hydroxyl group). Three infrared absorption spectra (Band 1: 3640–3390 cm−1, Band 2: 3000–2600 cm−1, Band 3: 2350 cm−1) indicate the presence of such hydroxyl groups. T. Abe et al. [14] reported that the Band 1 refers to protons that are not engaged in a hydrogen bond, while the Bands 2 and 3 refer to protons that are engaged in a hydrogen bond. They reported that the proton of the OH group that corresponds to the Band 1 is almost “immobile,” whereas the proton of the Band 2 is a “mobile” proton contributing to the conductivity of the glass thin films [26]. He also reported the following experimental equations for the phosphate glasses [18,19]: 2
Solid State Ionics 345 (2020) 115186
S.H. Lee, et al.
Fig. 1. Fabrication procedure of (a) BP and (b) BLAP glass thin films. Table 1 Phosphorus content, calculated proton concentration and log Ao values of BP and BLAP thin films. P/Ba, P/(Ba+La + Al)
BP 300 BP 400 BLAP 400
Stoichiometric
Precursor sol
After annealing
2.0 2.0 3.19
3.00 3.00 6.00
2.10 1.47 4.10
[H+] (mol/L)
log A0 (Scm−1mol−2l2)
Measured conductivity at 300 °C (S/cm)
55.9 45.5 87.3
−9.33 −9.53 −9.42
5.89 × 10−6 2.52 × 10−6 1.32 × 10−5
σ(417) = A 0 [H+]2
(1)
log A 0 = −0.0097 υOH + 17.1
(2)
μ = A 0 [H+]/e
(3)
wavenumber of hydrogen-bonded OH, e is the electronic charge and μ is proton mobility. The equation shows that the electrical conductivity, the proton concentration and the activation energy of phosphate glasses are closely related. Fig. 4 shows the FT-IR spectra for the fabricated BP and BLAP thin films. The glass thin films exhibited Band 1 around 3400–3700 cm−1, which is due to OeH group that is not engaged in a hydrogen bond.
where, A0 is the constant of proton mobility, σ is the conductivity at 417 K, [H+] is the mobile proton concentration, υOH is the FT-IR
Fig. 2. FE-SEM images of the surfaces and cross-sections of (a), (d) BP 300, (b), (e) BP 400, (c), (f) BLAP 400 deposited on Pt/Si substrates. 3
Solid State Ionics 345 (2020) 115186
S.H. Lee, et al.
Fig. 3. XRD patterns of (a) BP300, (b) BP400 and (c) BLAP 400.
Fig. 5. FT-IR spectra of Band 2; (a) BP 300, (b) BP 400, (c) BLAP 400.
temperature range of 150–350 °C is presented in Fig. 7. The maximum conductivities were obtained at the highest measuring temperature of 350 °C. However, the scattering of the data is so significant that it would be more reasonable to take the data at 300 °C close to the Arrhenius fitting lines. The obtained conductivity values at 300 °C are, 5.89 × 10−6 S/cm for BP 300, 2.52 × 10−6 S/cm for BP 400 and 1.32 × 10−5 S/cm for BLAP 400 as shown in Table 1. The proton concentrations of the glass thin films were calculated using Eq. (1), and the result is presented in Table 1. BLAP 400 exhibited the highest proton concentration of 87.3 mol/l compared to 55.9 mol/l of BP 300 and 45.5 mol/l of BP 400. Much higher proton concentrations than BP glass observed in BLAP 400 indicate that the high proton conductivity of BLAP glass in melt-quenched glasses can also be observed in thin films. The increase in the proton concentration of BP 300 in that of BP 400 is probably due to the high amount of molecular water and mobile protons at low annealing temperature. In contrast, the reported conductivity of the BLAP bulk glass is over 10−2 S/cm at 200 °C [19]. As a possible reason for the produced BLAP thin film to exhibit significantly lower conductivity compared to the bulk BLAP glass, it is conceivable that a very thin two-dimensional shape during conductivity measurement and annealing results in rapid water loss and high phosphorus evaporation. Although the BP and BLAP thin films showed much lower
Fig. 4. FT-IR spectra of the BP and BLAP glass thin films; (a) BP 300, (b) BP 400, (c) BLAP 400.
Around 2725–2745 cm−1, the characteristic peak of Band 2 was confirmed by fitting the zoomed-in spectra as shown in Fig. 5. The Band 2 wavenumber was 2725 cm−1 for BP 300 and 2745 cm−1 for BP 400. Using Eq. (2), log A0 values were calculated as −9.33, −9.53 and −9.42 for BP 300, BP 400 and BLAP 400, respectively. These values represent significantly higher values than those of the melt-quenched phosphate glasses. Fig. 6 shows the correlation between υOH and log A0 of BP, BLAP thin films and several representative melt-quenched phosphate glasses (MgO-P2O5, BeO-P2O5, etc.) [14,27–29]. The A0 values of the obtained thin films is very high compared to other phosphate glasses, indicating that they have high mobility of protons. Fig. 6 also shows that the silica glass has a lower proton mobility than typical phosphate glasses. This is because protons in the silica glass mostly exist in the immobile state [30].
3.3. Electrical properties The Arrhenius plot of the glass thin films measured in the 4
Solid State Ionics 345 (2020) 115186
S.H. Lee, et al.
Fig. 8. XPS spectra of the glass thin films; (a) BP 300, (b) BP 400, (c) BLAP 400. Fig. 6. Relation between log A0 in Eq. (1) and υOH in Eq. (2) in which the BP and BLAP glass thin films are indicated.
increase in activation energy when the measured temperature exceeds 250 °C. As described above, as one of the effects due to the thin film geometry, the measurement temperature is accompanied by the effect of heat treatment to change the structure of the thin film. The higher the measuring temperature, the lower the OH concentration and the higher the NBO concentration of the glass thin films, which may explain the scattered data and unstable change in activation energy with increasing temperature. It is reasonable to expect deterioration of the films during thermal cycles because the conductivity data shown in Fig. 7 appear to be accompanied by the heat treatment effect that changes the structure of the glass during the measurement. That is, rapid changes in the concentrations of OH and NBO and compositional change from severe phosphorous evaporation during thermal cycles can be considered as the main deterioration issues. Therefore, further research is needed to improve the stability of the glass thin films in the thermal cycle process. Fig. 8 shows the XPS spectra of the BLAP thin films. To investigate the effect of concentration of non-bridging oxygen (NBO) on proton conductivity, the XPS spectra O1s, representing oxygen binding energy of BP and BLAP thin films, are obtained and shown in Fig. 9. The relative amount of NBO and BO can be validated from the O1s spectra. The higher binding energy shown by the BO peak corresponds to approximately 533 eV, whereas a lower binding energy shown by the NBO peak corresponds to approximately 531 eV [28,33,34]. The peak intensity ratio of NBO/BO obtained from Fig. 9 are 1.41, 2.11 and 0.30 for BP 300, BP 400 and BLAP 400, respectively. Factors that are thought to affect proton mobility, namely the NBO/BO ratio and the previously mentioned log Ao, are slightly larger for BP 400 than for BP300. This result is in contrast to the observed conductivities. Furthermore, for BLAP 400, the NBO/BO ratio and log Ao are relatively small despite its high conductivity. Therefore, it seems that the proton concentration ([H+] in Table 1), which is strongly affected by residual water in glass network, appears to have dominant effect on proton conduction. From XPS quantitative analysis, a serious phosphorus loss during annealing was found. As the annealing temperature increased from 300 °C to 400 °C, the P/Ba ratio dropped sharply from 2.10 to 1.47 in BP films. Since the P/Ba of the starting solution was 3.0, 30.0% and 51.0% of phosphorus in BP 300 and 400, respectively, were volatilized during annealing. In BLAP 400, since P/(Ba + La + Al) of the starting solution was 6.0, 31.7% of phosphorus in the BLAP thin films was volatilized during annealing, which is a significantly reduced evaporation than in the case of BP 400. Thus, as the composition changes, the binding force to phosphorus in the glass network also changes, which seems to control the evaporation of phosphorus to some extent. The extent of serious phosphorus loss observed in this study has not been
Fig. 7. Conductivity of the glass thin films; (a) BP 300, (b) BP 400, (c) BLAP 400.
electrical conductivity than the bulk BP and BLAP glass, however, considering that the best conductivity of bulk perovskite proton-conductors (Y-doped BaCeO3) is about 10−4 S/cm at 200 °C [31], and conventional silica-based glasses are obviously ionic insulators, they still belong to the category of fast proton conductors. The calculated values of BP 300, 400 and BLAP 400 from the fitting lines were 0.28, 0.19 and 0.27 eV, respectively, and they are similar to the reported value of 0.17 eV of the bulk BLAP glass. Generally, higher heating temperature results in lower residual OH and higher phosphorus loss, and so activation energy increases. However, in this study, the activation energy of the BP 400 is lower than that of the BP 300, although the conductivity data for the BP 300 in Fig. 7 are significantly scattered. It is noteworthy that both BP 300 and 400 show a large 5
Solid State Ionics 345 (2020) 115186
S.H. Lee, et al.
4. Summary Fast proton-conducting BP and BLAP glass thin films, which have a high content of residual moisture and a high concentration of NBO as a medium in order to increase proton mobility, were fabricated through aqueous sol-gel processing. The fabricated 90–160 nm-thick glass thin films were homogenous, transparent and showed amorphous phase. As a result of XPS quantitative analysis, severe phosphorus losses of 30.0% to 51.0% were found during annealing and the P/Ba ratio dropped drastically as the annealing temperature increased. Therefore, consideration of process and compositional changes is necessary to avoid large evaporation of phosphorus in such phosphate thin films with nmscale thickness. That is, it is necessary to add an excess of phosphorus or to add an element, such as Nb, which strongly bonds with phosphorus. From the calculated proton concentrations of the glass thin films, BLAP 400 exhibited the highest proton concentration of 87.3 mol/l indicating that the high proton conductivity of BLAP glass in meltquenched glasses can also be observed in thin films. The increase in the proton concentration of BP 300 in that of BP 400 is probably due to the high amount of molecular water and mobile protons at low annealing temperature. And the proton-mobility constant Ao of the BP and BLAP glass thin films showed significantly higher values than those of the conventional melt-quenched glasses indicating that they have high mobility of protons. The electrical conductivities measured at 300 °C were 5.89 × 10−6 S/cm for BP300, 2.52 × 10−6 S/cm for BP400 and 1.32 × 10−5 S/cm for BLAP400. However, BLAP thin film exhibits significantly lower conductivity compared to that of the bulk BLAP glass. This is attributable to its very thin two-dimensional geometry, which promotes fast water loss and high phosphorus evaporation during conductivity measurement and annealing process. Therefore, stability issues of the glass thin films, including changes in concentrations of OH and NBO and changes in composition from phosphorous loss which are expected to occur during thermal cycles must be considered for application in high-temperature electrochemical devices. Considering that there is no stable solid electrolyte with superior proton conductivity in the intermediate temperature range of 200 °C to 500 °C, the low activation energy and the high proton conductivity of BP and BLAP glass thin films make them worthy of further study for a wide range of electrochemical applications. Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2016R1D1A1B01011819). Acknowledgement This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF2016R1D1A1B01011819). Fig. 9. XPS peaks of O1s in the glass thin films; (a) BP 300, (b) BP 400, (c) BLAP 400.
References [1] G. Alberti, M. Casciola, Solid State Ionics 145 (2001) 3–16. [2] S.W. Martin, J. Am. Ceram. Soc. 74 (1991) 1767–1784. [3] Y. Daiko, Y. Iwamoto, Handbook of Sol-Gel and Technology, (2018), pp. 2649–2661. [4] T. Omata, T. Yamaguchi, S. Tsukuda, T. Ishiyama, J. Nishii, T. Yamashita, H. Kawazoe, Phys. Chem. Chem. Phys. 21 (2019) 10744–10749. [5] Y. Daiko, S. Jung, S. Honda, Y. Iwamoto, Solid State Ionics 335 (2019) 151–155. [6] A. Chatterjee, A. Ghosh, Solid State Ionics 314 (2018) 1–8. [7] D.A. Boysen, T. Uda, C.R.I. Chisholm, S.M. Haile, Science 303 (2004) 68–70. [8] T. Tezuka, K. Tadanaga, A. Hayashi, M. Tatsumisago, Solid State Ionics 177 (2006) 2463–2466. [9] T. Kataoka, S. Tsukuda, T. Ishiyama, J. Nishii, T. Yamashita, H. Kawazoe, T. Omata, Phys. Chem. Chem. Phys. 19 (2019) 29669–29675. [10] Y. Abe, H. Shimakawa, J. Non-Cryst. Solids 51 (1982) 357–365.
reported in the previous studies on bulk phosphate glasses. In order to avoid the large phosphorus evaporation in the phosphate thin films with nm-scale thickness, the following methods can be considered. First, heat in a phosphorus atmosphere to prevent phosphorus evaporation. Second, add an excess amount of phosphorus to the precursor solution (as attempted in this study). Third, adding an element that binds strongly with phosphorus (for example, Nb [35]).
6
Solid State Ionics 345 (2020) 115186
S.H. Lee, et al.
[24] [25] [26] [27] [28]
[11] D.K. Gaikwad, M.I. Sayyed, S.N. Botewad, S.S. Obaid, Z.Y. Khattari, U.P. Gawai, F. Afaneh, M.D. Shirshat, P.P. Pawar, J. Non-Cryst. Solids 503–504 (2019) 158–168. [12] R.H. Doremus, J. Electrochem. Soc. 115 (1968) 181. [13] H. Namikawa, Y. Asahara, J. Ceram. Soc 74 (1966) 205–211. [14] Y. Abe, H. Hosono, Y. Ohta, L.L. Hench, Phys. Rev. B 38 (14) (1988) 10166–10169. [15] H. Namikawa, Y. Asahara, J. Ceram. Soc. Jpn. 74 (205) (1965) 9. [16] Y. Abe, M. Hayashi, T. Iwamoto, H. Sumi, L.L. Hench, J. Non-Cryst. Solids 351 (2005) 2138–2141. [17] H. Hosono, T. Kamae, Y. Abe, J. Am. Ceram. Soc. 72 (1989) 294. [18] Y. Abe, H. Hosono, T. Kamae, K. Kawashima, Phosphorus Sulfur Silicon 51 (1–4) (1990) 113–116. [19] S. Tung, B. Hwang, J. Membr. Sci. 241 (2004) 315–323. [20] Y. Takamatsu, Y. Daiko, S. Kohara, K. Suzuya, A. Mineshige, T. Yazawa, Solid State Ionics 245- 246 (2013) 19–23. [21] J. Kim, S.B. Park, Y. Park, Solid State Ionics 216 (2012) 15–18. [22] B.C. Lee, Y.J. Kwon, B.K. Ryu, J. Kor. Ceram. Soc 39 (265) (2002).
[29] [30] [31] [33] [34] [35]
7
Y. Abe, M. Takahashi, Chem. Phys. Lett. 411 (2005) 302–305. H. Scholze, Glastech. Ber. 32 (1959) 81. H. Hosono, K. Kawamura, H. Kawazoe, J. Appl. Phys. 81 (3) (1997) 1. T. Uma, M. Nogami, J. Membr. Sci. 280 (2006) 744–751. B.C. Sales, J.U. Otaigbe, G.H. Beall, L.A. Boatner, J.O. Ramey, J. Non-Cryst. Solids 226 (1998) 287–293. M. Jerroudi, L. Bih, M. Azrour, B. Manoun, I. Saadoune, P. Lazor, J. Inorg. Organomet. Polym. Mater. (2018) 1–11. Y. Abe, H. Hosono, O. Akita, L.L. Hench, J. Electrochem. Soc. 141 (6) (1994) L64. T. Norby, Solid State Ionics 125 (1) (1999). S. Donze, L. Montagne, J. Grimblot, L. Gengembre, G. Palavit, Phosphorus Res. Bull. 10 (1999) 509. J. Kang, Z. Chen, X. Zhu, S. Zhou, L. Zhou, Z. Wang, J. Wang, G.A. Khater, Y. Yue, J. Non-Cryst. Solids 503–504 (2019) 1–6. I. Nowak, M. Ziolek, Chem. Rev. 99 (12) (1999) 3603–3624.
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Proton-conducting BaO-P2O5 and BaO-La2O3-Al2O3-P2O5 glass thin films Suk Hee Lee, Sung Bum Park, Yong-il Park
⁎
T
School of Materials Science and Engineering, Kumoh National Institute of Technology, Gumi, Gyeongbuk, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: BaO-P2O5 BaO-La2O3-Al2O3-P2O5 Phosphate glass Thin film Proton conduction
BaO-P2O5 (BP) and BaO-La2O3-Al2O3-P2O5 (BLAP) glass thin films with a thickness of 80–160 nm, which have a high content of residual moisture and a high concentration of non-bridging oxygen (NBO) as a medium for proton movement, were fabricated through sol-gel spin coating using water as a solvent. All the fabricated glass thin films exhibited amorphous phase and glassy, defect-free surfaces. The thickness of the BP thin films decreased as the annealing temperature increased. The increase in the annealing temperature led to a decrease in NBO/BO ratio which is thought to affect proton mobility and an increase in mobile proton concentration. Therefore, it seems that high proton concentration by residual water appears to have a dominant effect on fast proton conduction. The calculated proton concentrations of the fabricated BP thin films were decreased from 55.9 mol/l to 45.5 mol/l as the annealing temperature increased from 300 °C to 400 °C. The conductivity of BP and BLAP thin films annealed at 400 °C were 2.52 × 10−6 S/cm and 1.32 × 10−5 S/cm, respectively, at 300 °C. BLAP thin film showed significantly improved conductivity compared to those of BP glass thin films due to its high proton concentration.
1. Introduction Proton-conducting phosphate glasses have a low glass transition temperature, low melting temperature, and high ion conductivity, and can be applied to various electrochemical devices as electrolytes of fuel cells, hydrogen separating membranes, and sensors [1–6]. Unlike silica glasses, phosphate glasses are reported to have protons with a high mobility [7–9]. The role of protons as a charge carrier in silica glasses has long been denied owing to the high bonding strength of OeH. Thus, research into the electrical conduction of glass has mostly been limited to alkaline ionic conduction and electronic conduction [10,11]. Typically, in oxide glass, a proton has a lower mobility than an alkali ion. Doremus et al. [12] reported that the mobility of Na+ in silica glass is 104 times higher than that of H+ possibly because strong OeH bonds exist in silica glasses. Hydrogen bonds, which are abundant in the glass, differ depending on the type of oxygen that is primarily bonded. The hydrogen bonding force is the weakest in the form of X-O-X (bridging oxygen: BO), while it becomes stronger in the form of X-O– (nonbridging oxygen: NBO). The intensity of a hydrogen bond depends on the form of the inner network of the bonding cation. The concentration of water that remains after the fabrication of a glass is 0.01–0.5 wt%. This remaining water results in the formation of an X-OH glass network, significantly affecting the properties of the glass. H. Namikawa et al. [13] identified the phenomenon of proton
⁎
conduction in a BaO-P2O5 (BP) glass. It was confirmed that protons become the main source of movement in the phosphate glass. The significant factors in determining the binary glass composition are the durability against water to obtain relative conductivity and the ease of changing the water content over a wide range. The dependency on these two factors was confirmed by a previous study on binary phosphate glasses [14]. The order of proton mobility in an alkali earth phosphate glass was reported to be arranged by the alkali component, that is, Ba > Sr > Ca > Mg [5]. Later, Y. Abe et al. reported a superionic conducting glass having a conductivity of 10−4 S/cm which maintain a high proton conductivity even at moderately hot temperatures of 200 °C–400 °C. [14,15] Among them, proton conductivity of 22BaO-2.5La2O3–0.5Al2O3-75P2O5 (BLAP) bulk glass, which was prepared by melt-quenching of a mixture of H3PO4 and metal carbonates at 700 °C for 30 min, was extremely high, about 10−2 S/cm at 200 °C due to residual water which exists as H2O at low temperatures and as OH group at temperatures above 100 °C [16]. The factors affecting the proton conduction of phosphate glass are the amount of residual water, proton concentration, and the amount of phosphorus inside the glass [17]. In the case of proton conduction without presence of water molecule, the conduction occurs by a hopping mechanism through OeH group. However, in glass containing water molecules, a proton quickly moves through the water molecules. That is, the water molecules act as a catalyst by lowering the activation
Corresponding author. E-mail address: [email protected] (Y.-i. Park).
https://doi.org/10.1016/j.ssi.2019.115186 Received 5 October 2019; Received in revised form 10 November 2019; Accepted 9 December 2019 0167-2738/ © 2019 Elsevier B.V. All rights reserved.
Solid State Ionics 345 (2020) 115186
S.H. Lee, et al.
Aldrich]. The BLAP thin films was fabricated through the processes shown in Fig. 1(b). Phosphorus pentoxide [P2O5, Sigma-Aldrich] was mixed in distilled water for 20 min until it dissolved and then combined with aluminum dihydrogen phosphate [AlH6O12P3, Alfa Aesar] before stirring for another 10 min. Subsequently, lanthanum nitrate hexahydrate and barium nitrate were added and mixed for 20 min. For the precursor solution, unlike the composition of the reported bulk BLAP glass of BaO:La2O3:Al2O3:P2O5 = 22:2.5:0.5:75 (in molar ratio) an excess amount of phosphorus was initially added to the precursor solution. The molar ratio of P/(Ba+La + Al) was fixed at 6.0 as the same as precursor solution for BP glass thin films, which is 1.88 times of P/ (Ba+La + Al) ratio of the reported BLAP bulk glass. The BLAP gel films were deposited by spin-coating the precursor solution on pretreated Ptcoated Si wafers for 30 s at 3000 rpm. Then the gel films were dried at 140 °C for 10 min and annealed at 400 °C for 10 min (from now on, BLAP 400). This drying-annealing process was repeated five times to produce glass thin films. In Table 1, phosphorus content, calculated proton concentration and log Ao values of BP and BLAP thin films are summarized. To observe the microstructure of the fabricated thin film, FE-SEM (FE-SEM, JEOL model JSM-6500F) was used and the thin film thickness was measured using post-deposition FE-SEM cross sectional images. To examine the chemical bonding structure and proton mobility, FT-IR (FT-IR, Vertex 80v) analysis was performed. XRD (SWXD, X-MAX/ 2000-PC, Rigaku) was used to check the crystal structure in the range of 2θ = 10–60° at the speed of 0.5°/min. To examine the electrical properties, a resistance measurement was carried out using an AC impedance analyzer (Solartron SI 1260, ULVAC KIKO Inc.), increasing temperature from 150 °C to 350 °C in though-plane direction. The measuring frequency range was 0.1–106 Hz, and the applied voltage was 10 mV. To analyze the components of the fabricated phosphate glass, an X-ray photoelectron spectrometer (XPS) (Thermo Fisher Scientific, Multilab-2000) was used.
energy for proton conduction. Thus, the conductivity becomes 3–4 times higher than that of the glass without water molecules [18]. Because protons in silica glass bind very strongly to oxygen, which limits their movement, resulting in lower proton conductivity. On the other hand, phosphate glass has a PO4 tetrahedral structure where the phosphorus cation and oxygen ion form a covalent bond depending on the composition. Since phosphorus cations tend to form structures of non-bridging oxygen (NBO) rather than tetrahedral structures, OeH bonds are weak, while hydrogen bonds are strong. As a result, both the proton mobility and the proton conductivity are increased [19]. That is, stronger hydrogen bonds between the proton and nearby NBO give rise to a weaker bonding force of the OH group, leading to a significant increase in proton mobility and an increase in proton conductivity. In this regard, it has been reported that when hydrolysis occurs in a P-O-P structure of a phosphate network, the number of P-OH groups increases, thereby increasing the concentration of mobile protons [20]. In addition, increasing the concentration of NBO forms strong hydrogen bonds, increasing the proton conductivity. Therefore, increased phosphate content results in high proton conductivity in the temperature range above 100 °C [21]. As a result, phosphate glass is expected to maintain a high proton conductivity even at intermediate temperature, i.e., 200–400 °C. Proton conductivity increases with phosphorus content, however, materials based on phosphate glass have some drawbacks. The degradation of chemical durability occurs due to the severe vaporization of phosphorus during annealing [20]. Since phosphorus easily volatilizes at high temperatures, causing a decrease in content during heat treatment and readily bonds with water, so the glass structure is very susceptible to breakage. Furthermore, the volatilization of phosphorus becomes more severe in the two-dimensional thin film form, which is preferred geometry as an electrolyte form in many electrochemical devices [22,21]. The effect of phosphorus volatilization on electrical properties has not been seriously reported for bulk phosphate glasses, but it can have a significant effect, especially for thin films with submicrometer thickness. In this study, BP glasses were fabricated in the form of sol-gel glass thin film which has very high applicability to electronic devices such as fuel cells and gas sensors, and the effects of phosphorus loss during annealing and their electrical properties were investigated. Furthermore, BLAP glass, which has been reported to have higher proton mobility than BP glass thin film was also fabricated and compared with the fabricated BP glass thin films. Water was used as a solvent to improve the proton conductivity by increasing the concentration of NBO in the phosphate glass.
3. Results and discussion 3.1. Microstructure Fig. 2 shows the surface and the cross-sectional FE-SEM images of BP and BLAP glass thin films. The glass thin films were all homogeneous and free of defects, and their thickness ranged from 90 to 160 nm. The difference in thickness between BP 300 and 400 is ascribable to an increase in additional density with increasing annealing temperature. XRD patterns of Fig. 3 show that all the prepared glass thin films have an amorphous phase. The increase in the intensity around 2θ = 10° is due to nanopores, which are typically observed in sol-gel-derived glasses [24].
2. Experimental Using barium nitrate (Ba(NO3)2) as a starting material, phosphoric acid was dissolved in distilled water to fabricate the precursor solution. As for composition of the precursor solution, an excess amount of phosphorus was initially added to the precursor solution. The P/Ba ratio was adjusted to 3.0 which is 1.5 times of the melt-quenched BaOP2O5 glass. After stirring barium nitrate (Ba(NO3)2) in distilled water for 20 min, the solution was mixed with a phosphoric acid solution and stirred again for 20 min. Then, spin coating was performed using the prepared solution on a Pt-coated Si-wafer substrate at 3000 rpm for 30 s to deposit a gel film. The gel film was dried at 140 °C for 10 min. In order to remove organic impurities and solvents that possibly remained in the thin film, heat- treatment was performed at 300 °C for 10 min and at 400 °C for 10 min. This process was repeated six times so that the total time taken for the annealing was 60 min at each 300 and 400 °C (from now on, BP 300 and 400, respectively.). Fig. 1(a) shows the process sequence for the BP glass thin films. To fabricate BLAP glass thin films, a clear precursor sol was synthesized using barium nitrate [Ba(NO3)2, Daejung], lanthanum nitrate hexahydrate [La(NO3)3·6H2O, Wako], aluminum dihydrogen phosphate [AlH6O12P3, Alfa Aesar], and phosphorus pentoxide [P2O5, Sigma-
3.2. Moisture and proton mobility Scholze et al. [25] studied the form and bonding state of the H2O in oxide glass and concluded that moisture within molecules does not exist in glass made by the typical melt-quenching process. Since protons in conventional silica glass bond to oxygen very strongly, hydrogen bonds are not formed, yielding low proton conductivity. However, in phosphate glass, the moisture remains in the form of OH (hydroxyl group). Three infrared absorption spectra (Band 1: 3640–3390 cm−1, Band 2: 3000–2600 cm−1, Band 3: 2350 cm−1) indicate the presence of such hydroxyl groups. T. Abe et al. [14] reported that the Band 1 refers to protons that are not engaged in a hydrogen bond, while the Bands 2 and 3 refer to protons that are engaged in a hydrogen bond. They reported that the proton of the OH group that corresponds to the Band 1 is almost “immobile,” whereas the proton of the Band 2 is a “mobile” proton contributing to the conductivity of the glass thin films [26]. He also reported the following experimental equations for the phosphate glasses [18,19]: 2
Solid State Ionics 345 (2020) 115186
S.H. Lee, et al.
Fig. 1. Fabrication procedure of (a) BP and (b) BLAP glass thin films. Table 1 Phosphorus content, calculated proton concentration and log Ao values of BP and BLAP thin films. P/Ba, P/(Ba+La + Al)
BP 300 BP 400 BLAP 400
Stoichiometric
Precursor sol
After annealing
2.0 2.0 3.19
3.00 3.00 6.00
2.10 1.47 4.10
[H+] (mol/L)
log A0 (Scm−1mol−2l2)
Measured conductivity at 300 °C (S/cm)
55.9 45.5 87.3
−9.33 −9.53 −9.42
5.89 × 10−6 2.52 × 10−6 1.32 × 10−5
σ(417) = A 0 [H+]2
(1)
log A 0 = −0.0097 υOH + 17.1
(2)
μ = A 0 [H+]/e
(3)
wavenumber of hydrogen-bonded OH, e is the electronic charge and μ is proton mobility. The equation shows that the electrical conductivity, the proton concentration and the activation energy of phosphate glasses are closely related. Fig. 4 shows the FT-IR spectra for the fabricated BP and BLAP thin films. The glass thin films exhibited Band 1 around 3400–3700 cm−1, which is due to OeH group that is not engaged in a hydrogen bond.
where, A0 is the constant of proton mobility, σ is the conductivity at 417 K, [H+] is the mobile proton concentration, υOH is the FT-IR
Fig. 2. FE-SEM images of the surfaces and cross-sections of (a), (d) BP 300, (b), (e) BP 400, (c), (f) BLAP 400 deposited on Pt/Si substrates. 3
Solid State Ionics 345 (2020) 115186
S.H. Lee, et al.
Fig. 3. XRD patterns of (a) BP300, (b) BP400 and (c) BLAP 400.
Fig. 5. FT-IR spectra of Band 2; (a) BP 300, (b) BP 400, (c) BLAP 400.
temperature range of 150–350 °C is presented in Fig. 7. The maximum conductivities were obtained at the highest measuring temperature of 350 °C. However, the scattering of the data is so significant that it would be more reasonable to take the data at 300 °C close to the Arrhenius fitting lines. The obtained conductivity values at 300 °C are, 5.89 × 10−6 S/cm for BP 300, 2.52 × 10−6 S/cm for BP 400 and 1.32 × 10−5 S/cm for BLAP 400 as shown in Table 1. The proton concentrations of the glass thin films were calculated using Eq. (1), and the result is presented in Table 1. BLAP 400 exhibited the highest proton concentration of 87.3 mol/l compared to 55.9 mol/l of BP 300 and 45.5 mol/l of BP 400. Much higher proton concentrations than BP glass observed in BLAP 400 indicate that the high proton conductivity of BLAP glass in melt-quenched glasses can also be observed in thin films. The increase in the proton concentration of BP 300 in that of BP 400 is probably due to the high amount of molecular water and mobile protons at low annealing temperature. In contrast, the reported conductivity of the BLAP bulk glass is over 10−2 S/cm at 200 °C [19]. As a possible reason for the produced BLAP thin film to exhibit significantly lower conductivity compared to the bulk BLAP glass, it is conceivable that a very thin two-dimensional shape during conductivity measurement and annealing results in rapid water loss and high phosphorus evaporation. Although the BP and BLAP thin films showed much lower
Fig. 4. FT-IR spectra of the BP and BLAP glass thin films; (a) BP 300, (b) BP 400, (c) BLAP 400.
Around 2725–2745 cm−1, the characteristic peak of Band 2 was confirmed by fitting the zoomed-in spectra as shown in Fig. 5. The Band 2 wavenumber was 2725 cm−1 for BP 300 and 2745 cm−1 for BP 400. Using Eq. (2), log A0 values were calculated as −9.33, −9.53 and −9.42 for BP 300, BP 400 and BLAP 400, respectively. These values represent significantly higher values than those of the melt-quenched phosphate glasses. Fig. 6 shows the correlation between υOH and log A0 of BP, BLAP thin films and several representative melt-quenched phosphate glasses (MgO-P2O5, BeO-P2O5, etc.) [14,27–29]. The A0 values of the obtained thin films is very high compared to other phosphate glasses, indicating that they have high mobility of protons. Fig. 6 also shows that the silica glass has a lower proton mobility than typical phosphate glasses. This is because protons in the silica glass mostly exist in the immobile state [30].
3.3. Electrical properties The Arrhenius plot of the glass thin films measured in the 4
Solid State Ionics 345 (2020) 115186
S.H. Lee, et al.
Fig. 8. XPS spectra of the glass thin films; (a) BP 300, (b) BP 400, (c) BLAP 400. Fig. 6. Relation between log A0 in Eq. (1) and υOH in Eq. (2) in which the BP and BLAP glass thin films are indicated.
increase in activation energy when the measured temperature exceeds 250 °C. As described above, as one of the effects due to the thin film geometry, the measurement temperature is accompanied by the effect of heat treatment to change the structure of the thin film. The higher the measuring temperature, the lower the OH concentration and the higher the NBO concentration of the glass thin films, which may explain the scattered data and unstable change in activation energy with increasing temperature. It is reasonable to expect deterioration of the films during thermal cycles because the conductivity data shown in Fig. 7 appear to be accompanied by the heat treatment effect that changes the structure of the glass during the measurement. That is, rapid changes in the concentrations of OH and NBO and compositional change from severe phosphorous evaporation during thermal cycles can be considered as the main deterioration issues. Therefore, further research is needed to improve the stability of the glass thin films in the thermal cycle process. Fig. 8 shows the XPS spectra of the BLAP thin films. To investigate the effect of concentration of non-bridging oxygen (NBO) on proton conductivity, the XPS spectra O1s, representing oxygen binding energy of BP and BLAP thin films, are obtained and shown in Fig. 9. The relative amount of NBO and BO can be validated from the O1s spectra. The higher binding energy shown by the BO peak corresponds to approximately 533 eV, whereas a lower binding energy shown by the NBO peak corresponds to approximately 531 eV [28,33,34]. The peak intensity ratio of NBO/BO obtained from Fig. 9 are 1.41, 2.11 and 0.30 for BP 300, BP 400 and BLAP 400, respectively. Factors that are thought to affect proton mobility, namely the NBO/BO ratio and the previously mentioned log Ao, are slightly larger for BP 400 than for BP300. This result is in contrast to the observed conductivities. Furthermore, for BLAP 400, the NBO/BO ratio and log Ao are relatively small despite its high conductivity. Therefore, it seems that the proton concentration ([H+] in Table 1), which is strongly affected by residual water in glass network, appears to have dominant effect on proton conduction. From XPS quantitative analysis, a serious phosphorus loss during annealing was found. As the annealing temperature increased from 300 °C to 400 °C, the P/Ba ratio dropped sharply from 2.10 to 1.47 in BP films. Since the P/Ba of the starting solution was 3.0, 30.0% and 51.0% of phosphorus in BP 300 and 400, respectively, were volatilized during annealing. In BLAP 400, since P/(Ba + La + Al) of the starting solution was 6.0, 31.7% of phosphorus in the BLAP thin films was volatilized during annealing, which is a significantly reduced evaporation than in the case of BP 400. Thus, as the composition changes, the binding force to phosphorus in the glass network also changes, which seems to control the evaporation of phosphorus to some extent. The extent of serious phosphorus loss observed in this study has not been
Fig. 7. Conductivity of the glass thin films; (a) BP 300, (b) BP 400, (c) BLAP 400.
electrical conductivity than the bulk BP and BLAP glass, however, considering that the best conductivity of bulk perovskite proton-conductors (Y-doped BaCeO3) is about 10−4 S/cm at 200 °C [31], and conventional silica-based glasses are obviously ionic insulators, they still belong to the category of fast proton conductors. The calculated values of BP 300, 400 and BLAP 400 from the fitting lines were 0.28, 0.19 and 0.27 eV, respectively, and they are similar to the reported value of 0.17 eV of the bulk BLAP glass. Generally, higher heating temperature results in lower residual OH and higher phosphorus loss, and so activation energy increases. However, in this study, the activation energy of the BP 400 is lower than that of the BP 300, although the conductivity data for the BP 300 in Fig. 7 are significantly scattered. It is noteworthy that both BP 300 and 400 show a large 5
Solid State Ionics 345 (2020) 115186
S.H. Lee, et al.
4. Summary Fast proton-conducting BP and BLAP glass thin films, which have a high content of residual moisture and a high concentration of NBO as a medium in order to increase proton mobility, were fabricated through aqueous sol-gel processing. The fabricated 90–160 nm-thick glass thin films were homogenous, transparent and showed amorphous phase. As a result of XPS quantitative analysis, severe phosphorus losses of 30.0% to 51.0% were found during annealing and the P/Ba ratio dropped drastically as the annealing temperature increased. Therefore, consideration of process and compositional changes is necessary to avoid large evaporation of phosphorus in such phosphate thin films with nmscale thickness. That is, it is necessary to add an excess of phosphorus or to add an element, such as Nb, which strongly bonds with phosphorus. From the calculated proton concentrations of the glass thin films, BLAP 400 exhibited the highest proton concentration of 87.3 mol/l indicating that the high proton conductivity of BLAP glass in meltquenched glasses can also be observed in thin films. The increase in the proton concentration of BP 300 in that of BP 400 is probably due to the high amount of molecular water and mobile protons at low annealing temperature. And the proton-mobility constant Ao of the BP and BLAP glass thin films showed significantly higher values than those of the conventional melt-quenched glasses indicating that they have high mobility of protons. The electrical conductivities measured at 300 °C were 5.89 × 10−6 S/cm for BP300, 2.52 × 10−6 S/cm for BP400 and 1.32 × 10−5 S/cm for BLAP400. However, BLAP thin film exhibits significantly lower conductivity compared to that of the bulk BLAP glass. This is attributable to its very thin two-dimensional geometry, which promotes fast water loss and high phosphorus evaporation during conductivity measurement and annealing process. Therefore, stability issues of the glass thin films, including changes in concentrations of OH and NBO and changes in composition from phosphorous loss which are expected to occur during thermal cycles must be considered for application in high-temperature electrochemical devices. Considering that there is no stable solid electrolyte with superior proton conductivity in the intermediate temperature range of 200 °C to 500 °C, the low activation energy and the high proton conductivity of BP and BLAP glass thin films make them worthy of further study for a wide range of electrochemical applications. Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2016R1D1A1B01011819). Acknowledgement This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF2016R1D1A1B01011819). Fig. 9. XPS peaks of O1s in the glass thin films; (a) BP 300, (b) BP 400, (c) BLAP 400.
References [1] G. Alberti, M. Casciola, Solid State Ionics 145 (2001) 3–16. [2] S.W. Martin, J. Am. Ceram. Soc. 74 (1991) 1767–1784. [3] Y. Daiko, Y. Iwamoto, Handbook of Sol-Gel and Technology, (2018), pp. 2649–2661. [4] T. Omata, T. Yamaguchi, S. Tsukuda, T. Ishiyama, J. Nishii, T. Yamashita, H. Kawazoe, Phys. Chem. Chem. Phys. 21 (2019) 10744–10749. [5] Y. Daiko, S. Jung, S. Honda, Y. Iwamoto, Solid State Ionics 335 (2019) 151–155. [6] A. Chatterjee, A. Ghosh, Solid State Ionics 314 (2018) 1–8. [7] D.A. Boysen, T. Uda, C.R.I. Chisholm, S.M. Haile, Science 303 (2004) 68–70. [8] T. Tezuka, K. Tadanaga, A. Hayashi, M. Tatsumisago, Solid State Ionics 177 (2006) 2463–2466. [9] T. Kataoka, S. Tsukuda, T. Ishiyama, J. Nishii, T. Yamashita, H. Kawazoe, T. Omata, Phys. Chem. Chem. Phys. 19 (2019) 29669–29675. [10] Y. Abe, H. Shimakawa, J. Non-Cryst. Solids 51 (1982) 357–365.
reported in the previous studies on bulk phosphate glasses. In order to avoid the large phosphorus evaporation in the phosphate thin films with nm-scale thickness, the following methods can be considered. First, heat in a phosphorus atmosphere to prevent phosphorus evaporation. Second, add an excess amount of phosphorus to the precursor solution (as attempted in this study). Third, adding an element that binds strongly with phosphorus (for example, Nb [35]).
6
Solid State Ionics 345 (2020) 115186
S.H. Lee, et al.
[24] [25] [26] [27] [28]
[11] D.K. Gaikwad, M.I. Sayyed, S.N. Botewad, S.S. Obaid, Z.Y. Khattari, U.P. Gawai, F. Afaneh, M.D. Shirshat, P.P. Pawar, J. Non-Cryst. Solids 503–504 (2019) 158–168. [12] R.H. Doremus, J. Electrochem. Soc. 115 (1968) 181. [13] H. Namikawa, Y. Asahara, J. Ceram. Soc 74 (1966) 205–211. [14] Y. Abe, H. Hosono, Y. Ohta, L.L. Hench, Phys. Rev. B 38 (14) (1988) 10166–10169. [15] H. Namikawa, Y. Asahara, J. Ceram. Soc. Jpn. 74 (205) (1965) 9. [16] Y. Abe, M. Hayashi, T. Iwamoto, H. Sumi, L.L. Hench, J. Non-Cryst. Solids 351 (2005) 2138–2141. [17] H. Hosono, T. Kamae, Y. Abe, J. Am. Ceram. Soc. 72 (1989) 294. [18] Y. Abe, H. Hosono, T. Kamae, K. Kawashima, Phosphorus Sulfur Silicon 51 (1–4) (1990) 113–116. [19] S. Tung, B. Hwang, J. Membr. Sci. 241 (2004) 315–323. [20] Y. Takamatsu, Y. Daiko, S. Kohara, K. Suzuya, A. Mineshige, T. Yazawa, Solid State Ionics 245- 246 (2013) 19–23. [21] J. Kim, S.B. Park, Y. Park, Solid State Ionics 216 (2012) 15–18. [22] B.C. Lee, Y.J. Kwon, B.K. Ryu, J. Kor. Ceram. Soc 39 (265) (2002).
[29] [30] [31] [33] [34] [35]
7
Y. Abe, M. Takahashi, Chem. Phys. Lett. 411 (2005) 302–305. H. Scholze, Glastech. Ber. 32 (1959) 81. H. Hosono, K. Kawamura, H. Kawazoe, J. Appl. Phys. 81 (3) (1997) 1. T. Uma, M. Nogami, J. Membr. Sci. 280 (2006) 744–751. B.C. Sales, J.U. Otaigbe, G.H. Beall, L.A. Boatner, J.O. Ramey, J. Non-Cryst. Solids 226 (1998) 287–293. M. Jerroudi, L. Bih, M. Azrour, B. Manoun, I. Saadoune, P. Lazor, J. Inorg. Organomet. Polym. Mater. (2018) 1–11. Y. Abe, H. Hosono, O. Akita, L.L. Hench, J. Electrochem. Soc. 141 (6) (1994) L64. T. Norby, Solid State Ionics 125 (1) (1999). S. Donze, L. Montagne, J. Grimblot, L. Gengembre, G. Palavit, Phosphorus Res. Bull. 10 (1999) 509. J. Kang, Z. Chen, X. Zhu, S. Zhou, L. Zhou, Z. Wang, J. Wang, G.A. Khater, Y. Yue, J. Non-Cryst. Solids 503–504 (2019) 1–6. I. Nowak, M. Ziolek, Chem. Rev. 99 (12) (1999) 3603–3624.