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Optical absorption and redox kinetics of YBa2Cu3O7 − δ thin films studied by optical in-situ spectroscopy
Solid State Ionics 315 (2018) 98–101
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Optical absorption and redox kinetics of YBa2Cu3O7 − δ thin films studied by optical in-situ spectroscopy
T
⁎
Jianmin Shia, , Michael Martensb, Frank Ludwigb, Klaus Dilgera, Klaus-Dieter Beckerc a
Institute of Joining and Welding, Technische Universität Braunschweig, Langer Kamp 8, 38106 Braunschweig, Germany Institute of Electrical Measurements and Fundamental Electrical Engineering, Technische Universität Braunschweig, Hans-Sommer-Str. 65, 38106 Braunschweig, Germany c Institute of Physical and Theoretical Chemistry, Technische Universität Braunschweig, Gaussstr. 17, 38106 Braunschweig, Germany b
A R T I C L E I N F O
A B S T R A C T
Keywords: Optical in-situ spectroscopy YBCO thin film Surface exchange reaction Redox kinetics
Optical absorption and redox kinetics of YBa2Cu3O7 − δ thin films in oxidizing (O2) and reducing (Ar/H2) atmospheres were studied at temperatures from 200 °C to 500 °C by means of in situ UV–vis-NIR optical spectroscopy. The optical spectra in oxidizing atmospheres are characterized by optical absorption of oxygen holes, O− (OI′), e. g., at about 450 nm at 200 °C, whereas those in reducing atmospheres are dominated by a band at about 600 nm due to electron hopping between Cu-ions. The fast redox processes of oxygen incorporation into and oxygen release from YBCO thin films induced by sudden changes in the ambient atmosphere between O2 and Ar/H2 are found to be controlled by surface exchange reaction. The oxygen surface exchange coefficients, kδ, determined from optical absorption relaxation experiments are about 4.96 × 10− 7 m/s for the oxidation process and about 4.85 × 10− 9 m/s for the reduction process at 500 °C. The temperature dependence of kδ yields an activation energy of about 0.3 eV for both oxidation and reduction processes in the studied temperature range. In addition, the rapid oxidation processes can be explained in terms of a high concentration of electrons in the reduced state of YBCO thin films, facilitating electron-transfer steps at the surface of YBCO film for oxygen exchange.
1. Introduction Since the discovery of superconductivity of copper-based complex oxides (YBa2Cu3O7 − δ, YBCO) with a transition temperature above 77 K [1], YBCO and related materials are a well-studied class of oxides. Their crystal structure, chemical composition, and electrical conductivity were intensively investigated in order to understand superconductivity mechanisms and to find new superconductors, as well as to exploit technical applications [1–4]. The non-stoichiometry of oxygen in YBCO, i.e. 0 ≤ δ ≤ 1 not only plays an important role in superconductivity but also makes this material a good model system to study defect chemistry, redox kinetics and oxygen transport properties. Oxygen incorporation into and release from YBCO in the form of single crystals, ceramics and thin films were previously investigated by using electrical conductivity measurements, oxygen tracer diffusion, thermogravimetric analysis and ellipsometry [5–15]. In most of the previous work it was concluded that redox processes of YBCO are dominated by oxygen diffusion in the bulk, thus assuming fast equilibration of oxygen-surface exchange reaction. Several studies showed, however, that the surface exchange reaction contributes to the overall redox ⁎
kinetics of YBCO thin films [5,11–13]. Optical methods have recently drawn much attention in the solid state ionic community to investigate gas-solid reactions and ionic diffusion in oxides [16–19]. One of the advantages of optical methods is their non-contact nature during measurements, which can avoid any unknown effects or limitations due to electrode-sample interfaces in electrical measurements. In the present work, optical in situ spectroscopy in the UV–vis-NIR range has been used to study the optical absorption of YBCO thin films in oxidizing and reducing conditions, and to monitor redox kinetics upon sudden changes in the ambient atmosphere of YBCO thin films between O2 and Ar + 5%H2 in the temperature range of 200 °C to 500 °C. It is found from the time-dependent optical absorption that redox processes of YBCO thin films are controlled by a surface exchange reaction step. The surface exchange coefficient, kδ, and its temperature dependence were also determined.
Corresponding author. E-mail address: [email protected] (J. Shi).
https://doi.org/10.1016/j.ssi.2017.12.002 Received 6 September 2017; Received in revised form 4 December 2017; Accepted 4 December 2017 0167-2738/ © 2017 Elsevier B.V. All rights reserved.
Solid State Ionics 315 (2018) 98–101
J. Shi et al.
precision optical spectrometer (Perkin Elmer, Lambda 900), a homemade high-temperature furnace and an atmosphere control unit [17,18]. Optical absorption relaxation experiments were performed at a fixed wavelength of 625 nm by switching the atmosphere between O2 and Ar + 5%H2 at a given temperature, in order to monitor redox processes of YBCO films. The time-dependent change in optical absorption was fitted to kinetic models involving a surface exchange reaction coefficient kδ and/or a chemical diffusion coefficient Dδ of oxygen. It turned out that the kinetic processes are dominated by the surface exchange reaction step, thus were fitted with an exponential function, Eq. (1).
At − A∞ c − c∞ kδ ∝ t = exp ⎡− × t⎤ ⎢ ⎥ A0 − A∞ c0 − c∞ ⎣ d ⎦
(a)
(1)
In case of very short re-equilibration times, Eq. (1) simplifies to a linear function,
At − A∞ c − c∞ kδ ∝ t =1− ×t A0 − A∞ c0 − c∞ d
(2)
Here, At and ct denote the optical absorbance and the concentration of absorption species in the YBCO thin film with the thickness of d, respectively, at time t. kδ is the surface exchange coefficient of oxygen. A0, A∞, c0 and c∞ are absorbance and concentration at t = 0 and t = ∞, respectively. 3. Results and discussion 3.1. Optical absorption spectra and point defects in oxidized and reduced states
(b)
Representative optical absorption spectra of YBCO thin films in O2 and in Ar + 5%H2 at 200 °C and 500 °C are shown in Fig. 2. Two distinct features in the optical spectra can be seen. One is the red-shift of the optical absorption edge due to the temperature effect. The shift amounts to about 40 nm for a temperature change from 200 °C to 500 °C for both the oxidized and reduced states. Another feature is the appearance of an optical absorption band at about 600 nm in the spectra of YBCO thin film in reducing atmospheres. The intensity of the absorption band increases with increasing temperature. This absorption band is attributed to the electronic transition of Cu-ions, particularly, to electron hopping between Cu-ions (Cu2 + or/and Cu3 +) due to chemical reduction. It is also noted that a weekly absorption band at about 450 nm found in the spectrum at 200 °C in O2 can be assigned to the optical absorption of holes trapped at oxide ions, like in other oxides [20], i.e., O− or OI′ in the Kröger-Vink notation of point defects, see below. However, this band is not resolved at higher temperatures,
Fig. 1. (a) Measurement of the Meissner-Ochsenfeld effect of the YBCO thin film. Due to the shielding currents emerging beneath the critical temperature of the film, electromagnetic induction is prevented between two separated coils [22]. The drop of the normalized induction voltage in dependence of the temperature is a measure of the film quality and suggests a critical temperature of TC = (87 ± 1) K. (b) Scanning electron micrograph of the surface morphology of the YBCO film indicating the typical c-axis epitaxial growth aligned to the a- and b-axis of a SrTiO3 substrate.
2. Materials and methods 2.1. Preparation of YBCO thin films The YBCO thin films were deposited on a (100) SrTiO3 single crystal substrate (10 × 10 × 0.5mm3) by means of pulsed laser deposition (PLD). The target was fabricated from stoichiometrically mixed Y2O3, BaCO3 and CuO powders and sintered in an oxygen atmosphere. For the deposition, a KrF excimer laser (wavelength 248 nm) and optimized parameters from Refs. [21,22] were used. An energy density of 2.0 J/ cm2 and pulse frequency of 3 Hz for 12 min resulted in a film thickness of around 200 nm. The oxygen partial pressure of 12 Pa and deposition temperature of 840 °C used are near the CuO-Cu2O-O2 equilibrium line for optimal in situ film growth of the tetragonal phase with δ ≅ 1. After post-deposition annealing in O2 at 400 °C, the superconducting orthorhombic phase (δ < 0.1) was obtained [2]. The critical temperature Tc of the YBCO film is (87 ± 1) K determined by using the Meissner-Ochsenfeld effect as shown in Fig. 1a [23]. SEM investigations confirm the expected epitaxial c-axis growth of YBCO films on the SrTiO3 substrate, Fig. 1b, as previously observed using XRD for YBCO thin films [22].
Absorbance (A. U.)
2.5 200 oC, O2 o
500 C, O2
2.0
200 oC, ArH2 500 oC, ArH2
1.5
1.0
0.5
400
500
600
700
800
Wavelength (nm) 2.2. Optical in-situ spectroscopy
Fig. 2. Optical absorption spectra of YBCO thin films in oxidizing and reducing atmospheres at high temperatures: black and red solid curves represent spectra at 200 °C in O2 and in Ar + 5%H2, green and blue dash lines show spectra at 500 °C in O2 and in Ar + 5%H2, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Optical absorption spectra of YBCO thin films in O2 and Ar + 5%H2 at high temperatures were measured in the range from 300 nm to 850 nm using an experimental setup consisting of a UV–vis-NIR high99
Solid State Ionics 315 (2018) 98–101
J. Shi et al.
because of the shift of the absorption edge. Considering the previous work on defect chemistry and electrical conductivity of YBCO [6,7] together with the optical absorption measurements in the present study, one can draw a preliminary conclusion about the majority of point defects in the present YBCO thin film. In the oxidizing state, the majority of point defects are electron holes, h ⋅, interstitial oxide ions (oxygen ions in the CueO chains), OI′', and holes trapped on oxide interstitials, OI', which is supported by the observation of an optical absorption peak due to O− in oxidizing atmospheres. The dopant Y3 + can also take part in charge compensation as positively charged ionic defects YBa⋅ in YBCO films. The absorption band of electron hopping between Cu2 + and Cu3 + ions in reducing atmospheres suggests the release of oxygen not only from oxygen interstitial sites in the CueO chains but also from oxygen lattice sites in the CueO plans. Thus the chemical reduction reaction process upon a large step change in oxygen partial pressure, e.g. from O2 to Ar/H2 proceeds via the following steps, i.e., Eq. (3) for the p-regime, and Eq. (4) or (5) for the n-regime. The defect complex between oxygen vacancies, VO⋅⋅ and electrons trapped at Cu ions, CuCu′ to form singly positive charged complex, (VO⋅⋅ CuCu′)⋅ is considered in Eq. (5).
O′I + h⋅ = O″I + 2h⋅ = 1/2O2 (g) + V×I
(3)
× × OO + 2CuCu = 1/2O2 (g) + V ⋅⋅O + 2Cu′Cu
(4)
× × OO + 2CuCu = 1/2O2 (g) + (V ⋅⋅OCu′Cu)⋅ + Cu′Cu
(5)
Normalized absorbance (A. U.)
Log k δ (m/s)
Linear fit Exponential fit
kδred = 4.85 E-9 m/s
0.2 0.0 0
20
40
60
80
Ea, red = (0.32 + 0.06) eV
1.2
1.4
1.6
1.8
2.0
2.2
2.4
data have been fitted to kinetic models involving the surface exchange coefficient, kδ, and/or the chemical diffusion coefficient, Dδ. The fitted lines shown in Fig. 3 represent the fits to the model containing the surface exchange reaction step only, Eq. (1) or (2). The quality of the fits indicates that the redox processes of YBCO films are controlled by the surface exchange reaction step. The surface exchange coefficients of oxygen, kδ, determined from optical relaxation experiments are 4.96 × 10− 7 m/s for the oxidation and 4.85 × 10− 9 m/s for the reduction process at 500 °C, respectively. The temperature dependence of kδ from the oxidation and reduction processes, respectively, is shown in Fig. 4 in an Arrhenius plot. The activation energy of about 0.3 eV determined optically for the c-epitaxial YBCO film in this work suggests a different process from oxygen diffusion dominating the redox kinetics of YBCO thin films, which points to the surface exchange reaction step. Lee et al. [14] also found an activation energy of 0.33 eV from in situ electrical measurements of YBCO thin films in the temperature range 450–550 °C. However, these authors considered this apparent energy barrier to be due to the oxygen diffusion plus a phase transition. Interestingly, a similar activation energy for the oxygen out-diffusion process in solution-deposited YBCO films was reported recently using electrical conductivity relaxation experiments [15], in which oxygen exchange at the surface of YBCO film was proposed to be the rate limiting step. The nearly identical activation energy of about 0.3 eV obtained for the surface exchange reaction in the oxidation and reduction process, suggests a common mechanism for oxygen surface exchange in oxidation and reduction. In addition, the surface exchange coefficients for oxidation, kδox, are about two orders in magnitude larger than those for reduction processes, kδred, in the measured temperature range. The different surface exchange reaction rates for the oxidation and reduction of YBCO thin films indicate that the defect states in the bulk and at the surface of the film play a main role in the surface exchange reaction of oxygen. For the gas-oxide surface exchange reaction, several elementary sub-steps have to be considered, such as (i) chemical adsorption and dissociation of O2 molecules at the surface of YBCO films, (ii) chargetransfer to oxygen adsorbates (O or O2) at an active surface site, (iii) surface diffusion and incorporation of O2 − ions in the lattice for oxygen transport in the bulk. It is not clear which elementary sub-step is the rate-determining one for the surface exchange reaction. The present observation that kδox is approximately two orders in magnitude larger than kδred means that a higher concentration of electrons in the reduced state of YBCO favors the incorporation of oxygen into YBCO thin films. Thus, our data support the assumption that the charge transfer between free electrons and adsorbate oxygen (oxygen atoms or molecules, O or O2) determines the rate of oxygen exchange reaction at the surface of YBCO films and also the overall redox processes. Further work combining several spectroscopic methods as well as surface-imaging
= 4.96E-7 m/s
0.4
-9
Fig. 4. Temperature dependence of the surface exchange coefficient kδ of YBCO thin films for oxidation and reduction processes. The activation energies for oxidation and reduction amount to about 0.34 and 0.32 eV, respectively.
kδox
0.6
Ea, ox = (0.34 + 0.10) eV
1000/T (K-1)
1.0 0.8
-8
-11
Chemical kinetics of gas-oxide reactions is characterized in general by a surface exchange step followed by a bulk diffusion process. In some cases, the surface exchange reaction controls the overall kinetic process, in which the diffusion process in the bulk is much faster, especially for thin films with a large amount of point-defects in their crystal lattice and with dislocations and interfaces in their microstructure. The transport of oxygen in YBCO is of anisotropic nature due to its noncubic crystal structure and due to the complexity of microstructure of ceramic samples [6]. The activation barrier of oxygen diffusion and conduction was reported thus in a relative large range, from about 1.0 eV to 2.1 eV [5–11]. Fig. 3 shows the normalized optical absorption at 625 nm of the YBCO thin film as a function of time in redox processes at 500 °C obtained from optical absorption relaxation experiments by a sudden change in the atmosphere of the film between O2 and Ar + 5% H2. The
Oxidation, Reduction,
Reduction Linear fit
-7
-10
3.2. Redox kinetics and surface exchange coefficient of oxygen
1.2
Oxidation,
-6
100
Time (s) Fig. 3. Normalized optical absorbance at 625 nm of YBCO thin films as a function of time at 500 °C upon a sudden change in the ambient atmosphere between O2 and Ar + 5% H2: full and open symbols are experimental data for oxidation and reduction, respectively. Dotted blue and solid red lines are fits to the surface exchange model, Eq. (1), yielding kδred or kδox according to Eq. (2). Note the faster oxidation than reduction process. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
100
Solid State Ionics 315 (2018) 98–101
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techniques under in-situ conditions is highly desired to verify the optical investigations.
Supercond. 13 (2003) 2882–2885. [6] J. Maier, P. Murugaraj, G. Pfundtener, Defect chemistry and anisotropic transport in YBa2Cu3O6–7, Solid State Ionics 40/41 (1990) 802–806. [7] J. Maier, G. Pfundtner, Defect chemistry of the high-Tc-superconductors, Adv. Mater. 3 (1991) 292–297. [8] Y. Ikuma, S. Akiyoshi, Diffusion of oxygen in YBa2Cu3O7 − y, J. Appl. Phys. 64 (1988) 3914–3917. [9] K.N. Tu, N.C. Yeh, S.I. Park, C.C. Tsuei, Diffusion of oxygen in superconducting YBa2Cu3O7 − δ ceramic oxides, Phys. Rev. B 39 (1989) 304–314. [10] W.A.M. Aarnink, R.P.J. IJsselstijn, J. Gao, A. van Silfhout, H. Rogalla, Oxygen transport in c-axis-oriented high-Tc superconducting YBa2Cu3O7 − δ thin films, studied in situ by in situ ellipsometry, Phys. Rev. B 45 (1992) 13002–13007. [11] K. Yamamoto, B.M. Lairson, J.C. Bravman, T.H. Geballe, Oxidation kinetics of YBa2Cu3O7 − δ thin films in the presence of atomic oxygen and molecular oxygen by in-situ resistivity measurements, J. Appl. Phys. 69 (1991) 7189–7201. [12] A. Gupta, Oxidation kinetics of YBa2Cu3O7 − δ thin films in different oxidizing ambients, Appl. Surf. Sci. 64 (1993) 103–110. [13] J. Hu, X. Hu, H. Hao, L. Gao, H. Song, D. Yang, A transient thermogravimetric study on the oxygen permeation at high temperature of the superconducting material YBaCu3O7 − δ, Solid State Ionics 176 (2004) 487–494. [14] S.H. Lee, S.C. Bae, J.K. Ku, H.J. Shin, Oxygen diffusion in epitaxial YBa2Cu3O7 − x thin films, Phys. Rev. B Condens. Matter 46 (1992) 9142–9146. [15] P. Cayado, C.F. Sanchez-Valdes, A. Stangl, M. Coll, P. Roura, A. Palau, T. Putig, X. Obradors, Untangling surface oxygen exchange effects in YBa2Cu3O6 + x thin films by electrical conductivity relaxation, Phys. Chem. Chem. Phys. 19 (2017) 14129–14140. [16] R. Merkle, J. Maier, How is oxygen incorporated into oxides? A comprehensive kinetic study of a simple solid-state reaction with SrTiO3 as a model material, Angew. Chem. 21 (2008) 3874–3894. [17] J. Shi, H. Fritze, G. Borchardt, K.-D. Becker, Defect chemistry, redox kinetics and chemical diffusion of lithium deficient lithium niobate, Phys. Chem. Chem. Phys. 13 (2011) 6925–6930. [18] J. Shi, D.-K. Lee, H.-I. Yoo, J. Janek, K.-D. Becker, Oxidation kinetics of N-doped TiO2 − δ thin films, Phys. Chem. Chem. Phys. 14 (2012) 2930–2937. [19] J.J. Kim, S.R. Bishop, D. Chen, H.L. Tuller, Investigation of nonstoichiometry in oxide thin films by simultaneous in situ optical absorption and chemical capacitance measurements: Pr-doped ceria, a case study, Chem. Mater. 26 (2014) 1374–1379. [20] O.F. Schirmer, O− bound small polarons in oxide materials, J. Phys. Condens. Matter 18 (2006) R667–R704. [21] M. Schilling, A. Guillaume, J.M. Scholtyssek, F. Ludwig, Design of experiments for highly reproducible pulsed laser deposition of YBa2Cu3O7 − δ, J. Phys. D. Appl. Phys. 47 (2014) 034008. [22] M. Schilling, F. Goerke, U. Merkt, Laser deposition of epitaxial YBa2Cu3O7 films with low droplet densities, Thin Solid Films 235 (1993) 202–209. [23] J.H. Claassen, M.E. Reeves, R.J. Soulen Jr., A contactless method for measurement of critical current density and critical temperature of superconducting films, Rev. Sci. Instrum. 62 (1991) 996–1004.
4. Conclusions Optical absorption in oxidizing and reducing atmospheres and redox kinetics of YBCO thin films were studied at temperatures from 200 °C to 500 °C by optical in-situ spectroscopy. The optical absorption in oxidizing atmospheres is characterized by a band of trapped holes, O− at about 450 nm, while a band of electron hoping between Cu2 + and Cu3 + ions at about 600 nm in reducing atmospheres. Redox kinetic processes monitored by optical absorption relaxation experiments were found to be dominated by the oxygen surface exchange reaction, with a value of surface exchange coefficient, kδ of 4.96 × 10− 7 m/s for the oxidation process and of 4.85 × 10− 9 m/s for the reduction process, respectively, at 500 °C. The temperature dependence of kδ yields an activation energy of about 0.3 eV for both processes. The faster oxidation than reduction processes observed in this work favors the elementary step involving electron transfer at the YBCO surface being the rate-limiting step in the oxygen surface exchange reaction. Acknowledgement J. Shi acknowledges the financial support from German Research Foundation (DFG, SH802/2-1). References [1] M.K. Wu, J.R. Ashburn, C. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang, C.W. Chu, Superconductivity at 93 K in a new mixed-phase Y-Ba-Cu-O compound system at ambient pressure, Phys. Rev. Lett. 58 (1987) 908–910. [2] R. Feenstra, T.B. Lindemer, J.D. Budai, M.D. Galloway, Effect of oxygen pressure on the synthesis of YBa2Cu3O7 − x thin films by post-deposition annealing, J. Appl. Phys. 69 (1991) 6569–6585. [3] Z.Z. Sheng, A.M. Hermann, Bulk superconductivity at 120 K in the TI-Ca/Ba-Cu-O system, Nature 332 (1988) 138–139. [4] R. Mankowsky, A. Subedi, M. Först, S.O. Mariager, M. Chollet, et al., Nonlinear lattice dynamics as a basis for enhanced superconductivity in YBa2Cu3O6.5, Nature 516 (2014) 71–73. [5] L. Chen, C.L. Chen, A.J. Jacobson, Electrical conductivity relaxation studies of oxygen transport in epitaxial YBa2Cu3O7 − δ thin films, IEEE Trans. Appl.
101
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Optical absorption and redox kinetics of YBa2Cu3O7 − δ thin films studied by optical in-situ spectroscopy
T
⁎
Jianmin Shia, , Michael Martensb, Frank Ludwigb, Klaus Dilgera, Klaus-Dieter Beckerc a
Institute of Joining and Welding, Technische Universität Braunschweig, Langer Kamp 8, 38106 Braunschweig, Germany Institute of Electrical Measurements and Fundamental Electrical Engineering, Technische Universität Braunschweig, Hans-Sommer-Str. 65, 38106 Braunschweig, Germany c Institute of Physical and Theoretical Chemistry, Technische Universität Braunschweig, Gaussstr. 17, 38106 Braunschweig, Germany b
A R T I C L E I N F O
A B S T R A C T
Keywords: Optical in-situ spectroscopy YBCO thin film Surface exchange reaction Redox kinetics
Optical absorption and redox kinetics of YBa2Cu3O7 − δ thin films in oxidizing (O2) and reducing (Ar/H2) atmospheres were studied at temperatures from 200 °C to 500 °C by means of in situ UV–vis-NIR optical spectroscopy. The optical spectra in oxidizing atmospheres are characterized by optical absorption of oxygen holes, O− (OI′), e. g., at about 450 nm at 200 °C, whereas those in reducing atmospheres are dominated by a band at about 600 nm due to electron hopping between Cu-ions. The fast redox processes of oxygen incorporation into and oxygen release from YBCO thin films induced by sudden changes in the ambient atmosphere between O2 and Ar/H2 are found to be controlled by surface exchange reaction. The oxygen surface exchange coefficients, kδ, determined from optical absorption relaxation experiments are about 4.96 × 10− 7 m/s for the oxidation process and about 4.85 × 10− 9 m/s for the reduction process at 500 °C. The temperature dependence of kδ yields an activation energy of about 0.3 eV for both oxidation and reduction processes in the studied temperature range. In addition, the rapid oxidation processes can be explained in terms of a high concentration of electrons in the reduced state of YBCO thin films, facilitating electron-transfer steps at the surface of YBCO film for oxygen exchange.
1. Introduction Since the discovery of superconductivity of copper-based complex oxides (YBa2Cu3O7 − δ, YBCO) with a transition temperature above 77 K [1], YBCO and related materials are a well-studied class of oxides. Their crystal structure, chemical composition, and electrical conductivity were intensively investigated in order to understand superconductivity mechanisms and to find new superconductors, as well as to exploit technical applications [1–4]. The non-stoichiometry of oxygen in YBCO, i.e. 0 ≤ δ ≤ 1 not only plays an important role in superconductivity but also makes this material a good model system to study defect chemistry, redox kinetics and oxygen transport properties. Oxygen incorporation into and release from YBCO in the form of single crystals, ceramics and thin films were previously investigated by using electrical conductivity measurements, oxygen tracer diffusion, thermogravimetric analysis and ellipsometry [5–15]. In most of the previous work it was concluded that redox processes of YBCO are dominated by oxygen diffusion in the bulk, thus assuming fast equilibration of oxygen-surface exchange reaction. Several studies showed, however, that the surface exchange reaction contributes to the overall redox ⁎
kinetics of YBCO thin films [5,11–13]. Optical methods have recently drawn much attention in the solid state ionic community to investigate gas-solid reactions and ionic diffusion in oxides [16–19]. One of the advantages of optical methods is their non-contact nature during measurements, which can avoid any unknown effects or limitations due to electrode-sample interfaces in electrical measurements. In the present work, optical in situ spectroscopy in the UV–vis-NIR range has been used to study the optical absorption of YBCO thin films in oxidizing and reducing conditions, and to monitor redox kinetics upon sudden changes in the ambient atmosphere of YBCO thin films between O2 and Ar + 5%H2 in the temperature range of 200 °C to 500 °C. It is found from the time-dependent optical absorption that redox processes of YBCO thin films are controlled by a surface exchange reaction step. The surface exchange coefficient, kδ, and its temperature dependence were also determined.
Corresponding author. E-mail address: [email protected] (J. Shi).
https://doi.org/10.1016/j.ssi.2017.12.002 Received 6 September 2017; Received in revised form 4 December 2017; Accepted 4 December 2017 0167-2738/ © 2017 Elsevier B.V. All rights reserved.
Solid State Ionics 315 (2018) 98–101
J. Shi et al.
precision optical spectrometer (Perkin Elmer, Lambda 900), a homemade high-temperature furnace and an atmosphere control unit [17,18]. Optical absorption relaxation experiments were performed at a fixed wavelength of 625 nm by switching the atmosphere between O2 and Ar + 5%H2 at a given temperature, in order to monitor redox processes of YBCO films. The time-dependent change in optical absorption was fitted to kinetic models involving a surface exchange reaction coefficient kδ and/or a chemical diffusion coefficient Dδ of oxygen. It turned out that the kinetic processes are dominated by the surface exchange reaction step, thus were fitted with an exponential function, Eq. (1).
At − A∞ c − c∞ kδ ∝ t = exp ⎡− × t⎤ ⎢ ⎥ A0 − A∞ c0 − c∞ ⎣ d ⎦
(a)
(1)
In case of very short re-equilibration times, Eq. (1) simplifies to a linear function,
At − A∞ c − c∞ kδ ∝ t =1− ×t A0 − A∞ c0 − c∞ d
(2)
Here, At and ct denote the optical absorbance and the concentration of absorption species in the YBCO thin film with the thickness of d, respectively, at time t. kδ is the surface exchange coefficient of oxygen. A0, A∞, c0 and c∞ are absorbance and concentration at t = 0 and t = ∞, respectively. 3. Results and discussion 3.1. Optical absorption spectra and point defects in oxidized and reduced states
(b)
Representative optical absorption spectra of YBCO thin films in O2 and in Ar + 5%H2 at 200 °C and 500 °C are shown in Fig. 2. Two distinct features in the optical spectra can be seen. One is the red-shift of the optical absorption edge due to the temperature effect. The shift amounts to about 40 nm for a temperature change from 200 °C to 500 °C for both the oxidized and reduced states. Another feature is the appearance of an optical absorption band at about 600 nm in the spectra of YBCO thin film in reducing atmospheres. The intensity of the absorption band increases with increasing temperature. This absorption band is attributed to the electronic transition of Cu-ions, particularly, to electron hopping between Cu-ions (Cu2 + or/and Cu3 +) due to chemical reduction. It is also noted that a weekly absorption band at about 450 nm found in the spectrum at 200 °C in O2 can be assigned to the optical absorption of holes trapped at oxide ions, like in other oxides [20], i.e., O− or OI′ in the Kröger-Vink notation of point defects, see below. However, this band is not resolved at higher temperatures,
Fig. 1. (a) Measurement of the Meissner-Ochsenfeld effect of the YBCO thin film. Due to the shielding currents emerging beneath the critical temperature of the film, electromagnetic induction is prevented between two separated coils [22]. The drop of the normalized induction voltage in dependence of the temperature is a measure of the film quality and suggests a critical temperature of TC = (87 ± 1) K. (b) Scanning electron micrograph of the surface morphology of the YBCO film indicating the typical c-axis epitaxial growth aligned to the a- and b-axis of a SrTiO3 substrate.
2. Materials and methods 2.1. Preparation of YBCO thin films The YBCO thin films were deposited on a (100) SrTiO3 single crystal substrate (10 × 10 × 0.5mm3) by means of pulsed laser deposition (PLD). The target was fabricated from stoichiometrically mixed Y2O3, BaCO3 and CuO powders and sintered in an oxygen atmosphere. For the deposition, a KrF excimer laser (wavelength 248 nm) and optimized parameters from Refs. [21,22] were used. An energy density of 2.0 J/ cm2 and pulse frequency of 3 Hz for 12 min resulted in a film thickness of around 200 nm. The oxygen partial pressure of 12 Pa and deposition temperature of 840 °C used are near the CuO-Cu2O-O2 equilibrium line for optimal in situ film growth of the tetragonal phase with δ ≅ 1. After post-deposition annealing in O2 at 400 °C, the superconducting orthorhombic phase (δ < 0.1) was obtained [2]. The critical temperature Tc of the YBCO film is (87 ± 1) K determined by using the Meissner-Ochsenfeld effect as shown in Fig. 1a [23]. SEM investigations confirm the expected epitaxial c-axis growth of YBCO films on the SrTiO3 substrate, Fig. 1b, as previously observed using XRD for YBCO thin films [22].
Absorbance (A. U.)
2.5 200 oC, O2 o
500 C, O2
2.0
200 oC, ArH2 500 oC, ArH2
1.5
1.0
0.5
400
500
600
700
800
Wavelength (nm) 2.2. Optical in-situ spectroscopy
Fig. 2. Optical absorption spectra of YBCO thin films in oxidizing and reducing atmospheres at high temperatures: black and red solid curves represent spectra at 200 °C in O2 and in Ar + 5%H2, green and blue dash lines show spectra at 500 °C in O2 and in Ar + 5%H2, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Optical absorption spectra of YBCO thin films in O2 and Ar + 5%H2 at high temperatures were measured in the range from 300 nm to 850 nm using an experimental setup consisting of a UV–vis-NIR high99
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because of the shift of the absorption edge. Considering the previous work on defect chemistry and electrical conductivity of YBCO [6,7] together with the optical absorption measurements in the present study, one can draw a preliminary conclusion about the majority of point defects in the present YBCO thin film. In the oxidizing state, the majority of point defects are electron holes, h ⋅, interstitial oxide ions (oxygen ions in the CueO chains), OI′', and holes trapped on oxide interstitials, OI', which is supported by the observation of an optical absorption peak due to O− in oxidizing atmospheres. The dopant Y3 + can also take part in charge compensation as positively charged ionic defects YBa⋅ in YBCO films. The absorption band of electron hopping between Cu2 + and Cu3 + ions in reducing atmospheres suggests the release of oxygen not only from oxygen interstitial sites in the CueO chains but also from oxygen lattice sites in the CueO plans. Thus the chemical reduction reaction process upon a large step change in oxygen partial pressure, e.g. from O2 to Ar/H2 proceeds via the following steps, i.e., Eq. (3) for the p-regime, and Eq. (4) or (5) for the n-regime. The defect complex between oxygen vacancies, VO⋅⋅ and electrons trapped at Cu ions, CuCu′ to form singly positive charged complex, (VO⋅⋅ CuCu′)⋅ is considered in Eq. (5).
O′I + h⋅ = O″I + 2h⋅ = 1/2O2 (g) + V×I
(3)
× × OO + 2CuCu = 1/2O2 (g) + V ⋅⋅O + 2Cu′Cu
(4)
× × OO + 2CuCu = 1/2O2 (g) + (V ⋅⋅OCu′Cu)⋅ + Cu′Cu
(5)
Normalized absorbance (A. U.)
Log k δ (m/s)
Linear fit Exponential fit
kδred = 4.85 E-9 m/s
0.2 0.0 0
20
40
60
80
Ea, red = (0.32 + 0.06) eV
1.2
1.4
1.6
1.8
2.0
2.2
2.4
data have been fitted to kinetic models involving the surface exchange coefficient, kδ, and/or the chemical diffusion coefficient, Dδ. The fitted lines shown in Fig. 3 represent the fits to the model containing the surface exchange reaction step only, Eq. (1) or (2). The quality of the fits indicates that the redox processes of YBCO films are controlled by the surface exchange reaction step. The surface exchange coefficients of oxygen, kδ, determined from optical relaxation experiments are 4.96 × 10− 7 m/s for the oxidation and 4.85 × 10− 9 m/s for the reduction process at 500 °C, respectively. The temperature dependence of kδ from the oxidation and reduction processes, respectively, is shown in Fig. 4 in an Arrhenius plot. The activation energy of about 0.3 eV determined optically for the c-epitaxial YBCO film in this work suggests a different process from oxygen diffusion dominating the redox kinetics of YBCO thin films, which points to the surface exchange reaction step. Lee et al. [14] also found an activation energy of 0.33 eV from in situ electrical measurements of YBCO thin films in the temperature range 450–550 °C. However, these authors considered this apparent energy barrier to be due to the oxygen diffusion plus a phase transition. Interestingly, a similar activation energy for the oxygen out-diffusion process in solution-deposited YBCO films was reported recently using electrical conductivity relaxation experiments [15], in which oxygen exchange at the surface of YBCO film was proposed to be the rate limiting step. The nearly identical activation energy of about 0.3 eV obtained for the surface exchange reaction in the oxidation and reduction process, suggests a common mechanism for oxygen surface exchange in oxidation and reduction. In addition, the surface exchange coefficients for oxidation, kδox, are about two orders in magnitude larger than those for reduction processes, kδred, in the measured temperature range. The different surface exchange reaction rates for the oxidation and reduction of YBCO thin films indicate that the defect states in the bulk and at the surface of the film play a main role in the surface exchange reaction of oxygen. For the gas-oxide surface exchange reaction, several elementary sub-steps have to be considered, such as (i) chemical adsorption and dissociation of O2 molecules at the surface of YBCO films, (ii) chargetransfer to oxygen adsorbates (O or O2) at an active surface site, (iii) surface diffusion and incorporation of O2 − ions in the lattice for oxygen transport in the bulk. It is not clear which elementary sub-step is the rate-determining one for the surface exchange reaction. The present observation that kδox is approximately two orders in magnitude larger than kδred means that a higher concentration of electrons in the reduced state of YBCO favors the incorporation of oxygen into YBCO thin films. Thus, our data support the assumption that the charge transfer between free electrons and adsorbate oxygen (oxygen atoms or molecules, O or O2) determines the rate of oxygen exchange reaction at the surface of YBCO films and also the overall redox processes. Further work combining several spectroscopic methods as well as surface-imaging
= 4.96E-7 m/s
0.4
-9
Fig. 4. Temperature dependence of the surface exchange coefficient kδ of YBCO thin films for oxidation and reduction processes. The activation energies for oxidation and reduction amount to about 0.34 and 0.32 eV, respectively.
kδox
0.6
Ea, ox = (0.34 + 0.10) eV
1000/T (K-1)
1.0 0.8
-8
-11
Chemical kinetics of gas-oxide reactions is characterized in general by a surface exchange step followed by a bulk diffusion process. In some cases, the surface exchange reaction controls the overall kinetic process, in which the diffusion process in the bulk is much faster, especially for thin films with a large amount of point-defects in their crystal lattice and with dislocations and interfaces in their microstructure. The transport of oxygen in YBCO is of anisotropic nature due to its noncubic crystal structure and due to the complexity of microstructure of ceramic samples [6]. The activation barrier of oxygen diffusion and conduction was reported thus in a relative large range, from about 1.0 eV to 2.1 eV [5–11]. Fig. 3 shows the normalized optical absorption at 625 nm of the YBCO thin film as a function of time in redox processes at 500 °C obtained from optical absorption relaxation experiments by a sudden change in the atmosphere of the film between O2 and Ar + 5% H2. The
Oxidation, Reduction,
Reduction Linear fit
-7
-10
3.2. Redox kinetics and surface exchange coefficient of oxygen
1.2
Oxidation,
-6
100
Time (s) Fig. 3. Normalized optical absorbance at 625 nm of YBCO thin films as a function of time at 500 °C upon a sudden change in the ambient atmosphere between O2 and Ar + 5% H2: full and open symbols are experimental data for oxidation and reduction, respectively. Dotted blue and solid red lines are fits to the surface exchange model, Eq. (1), yielding kδred or kδox according to Eq. (2). Note the faster oxidation than reduction process. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
100
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techniques under in-situ conditions is highly desired to verify the optical investigations.
Supercond. 13 (2003) 2882–2885. [6] J. Maier, P. Murugaraj, G. Pfundtener, Defect chemistry and anisotropic transport in YBa2Cu3O6–7, Solid State Ionics 40/41 (1990) 802–806. [7] J. Maier, G. Pfundtner, Defect chemistry of the high-Tc-superconductors, Adv. Mater. 3 (1991) 292–297. [8] Y. Ikuma, S. Akiyoshi, Diffusion of oxygen in YBa2Cu3O7 − y, J. Appl. Phys. 64 (1988) 3914–3917. [9] K.N. Tu, N.C. Yeh, S.I. Park, C.C. Tsuei, Diffusion of oxygen in superconducting YBa2Cu3O7 − δ ceramic oxides, Phys. Rev. B 39 (1989) 304–314. [10] W.A.M. Aarnink, R.P.J. IJsselstijn, J. Gao, A. van Silfhout, H. Rogalla, Oxygen transport in c-axis-oriented high-Tc superconducting YBa2Cu3O7 − δ thin films, studied in situ by in situ ellipsometry, Phys. Rev. B 45 (1992) 13002–13007. [11] K. Yamamoto, B.M. Lairson, J.C. Bravman, T.H. Geballe, Oxidation kinetics of YBa2Cu3O7 − δ thin films in the presence of atomic oxygen and molecular oxygen by in-situ resistivity measurements, J. Appl. Phys. 69 (1991) 7189–7201. [12] A. Gupta, Oxidation kinetics of YBa2Cu3O7 − δ thin films in different oxidizing ambients, Appl. Surf. Sci. 64 (1993) 103–110. [13] J. Hu, X. Hu, H. Hao, L. Gao, H. Song, D. Yang, A transient thermogravimetric study on the oxygen permeation at high temperature of the superconducting material YBaCu3O7 − δ, Solid State Ionics 176 (2004) 487–494. [14] S.H. Lee, S.C. Bae, J.K. Ku, H.J. Shin, Oxygen diffusion in epitaxial YBa2Cu3O7 − x thin films, Phys. Rev. B Condens. Matter 46 (1992) 9142–9146. [15] P. Cayado, C.F. Sanchez-Valdes, A. Stangl, M. Coll, P. Roura, A. Palau, T. Putig, X. Obradors, Untangling surface oxygen exchange effects in YBa2Cu3O6 + x thin films by electrical conductivity relaxation, Phys. Chem. Chem. Phys. 19 (2017) 14129–14140. [16] R. Merkle, J. Maier, How is oxygen incorporated into oxides? A comprehensive kinetic study of a simple solid-state reaction with SrTiO3 as a model material, Angew. Chem. 21 (2008) 3874–3894. [17] J. Shi, H. Fritze, G. Borchardt, K.-D. Becker, Defect chemistry, redox kinetics and chemical diffusion of lithium deficient lithium niobate, Phys. Chem. Chem. Phys. 13 (2011) 6925–6930. [18] J. Shi, D.-K. Lee, H.-I. Yoo, J. Janek, K.-D. Becker, Oxidation kinetics of N-doped TiO2 − δ thin films, Phys. Chem. Chem. Phys. 14 (2012) 2930–2937. [19] J.J. Kim, S.R. Bishop, D. Chen, H.L. Tuller, Investigation of nonstoichiometry in oxide thin films by simultaneous in situ optical absorption and chemical capacitance measurements: Pr-doped ceria, a case study, Chem. Mater. 26 (2014) 1374–1379. [20] O.F. Schirmer, O− bound small polarons in oxide materials, J. Phys. Condens. Matter 18 (2006) R667–R704. [21] M. Schilling, A. Guillaume, J.M. Scholtyssek, F. Ludwig, Design of experiments for highly reproducible pulsed laser deposition of YBa2Cu3O7 − δ, J. Phys. D. Appl. Phys. 47 (2014) 034008. [22] M. Schilling, F. Goerke, U. Merkt, Laser deposition of epitaxial YBa2Cu3O7 films with low droplet densities, Thin Solid Films 235 (1993) 202–209. [23] J.H. Claassen, M.E. Reeves, R.J. Soulen Jr., A contactless method for measurement of critical current density and critical temperature of superconducting films, Rev. Sci. Instrum. 62 (1991) 996–1004.
4. Conclusions Optical absorption in oxidizing and reducing atmospheres and redox kinetics of YBCO thin films were studied at temperatures from 200 °C to 500 °C by optical in-situ spectroscopy. The optical absorption in oxidizing atmospheres is characterized by a band of trapped holes, O− at about 450 nm, while a band of electron hoping between Cu2 + and Cu3 + ions at about 600 nm in reducing atmospheres. Redox kinetic processes monitored by optical absorption relaxation experiments were found to be dominated by the oxygen surface exchange reaction, with a value of surface exchange coefficient, kδ of 4.96 × 10− 7 m/s for the oxidation process and of 4.85 × 10− 9 m/s for the reduction process, respectively, at 500 °C. The temperature dependence of kδ yields an activation energy of about 0.3 eV for both processes. The faster oxidation than reduction processes observed in this work favors the elementary step involving electron transfer at the YBCO surface being the rate-limiting step in the oxygen surface exchange reaction. Acknowledgement J. Shi acknowledges the financial support from German Research Foundation (DFG, SH802/2-1). References [1] M.K. Wu, J.R. Ashburn, C. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang, C.W. Chu, Superconductivity at 93 K in a new mixed-phase Y-Ba-Cu-O compound system at ambient pressure, Phys. Rev. Lett. 58 (1987) 908–910. [2] R. Feenstra, T.B. Lindemer, J.D. Budai, M.D. Galloway, Effect of oxygen pressure on the synthesis of YBa2Cu3O7 − x thin films by post-deposition annealing, J. Appl. Phys. 69 (1991) 6569–6585. [3] Z.Z. Sheng, A.M. Hermann, Bulk superconductivity at 120 K in the TI-Ca/Ba-Cu-O system, Nature 332 (1988) 138–139. [4] R. Mankowsky, A. Subedi, M. Först, S.O. Mariager, M. Chollet, et al., Nonlinear lattice dynamics as a basis for enhanced superconductivity in YBa2Cu3O6.5, Nature 516 (2014) 71–73. [5] L. Chen, C.L. Chen, A.J. Jacobson, Electrical conductivity relaxation studies of oxygen transport in epitaxial YBa2Cu3O7 − δ thin films, IEEE Trans. Appl.
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