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Epitaxial Yttria-Stabilized Zirconia on Muscovite for Flexible Transparent Ionic Conductors Ping-Chun Wu, Yu-Ping Lin, Yung-Hsiang Juan, YaoMing Wang, Thi Hien Do, Horng-Yi Chang, and Ying-Hao Chu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01698 • Publication Date (Web): 20 Nov 2018 Downloaded from http://pubs.acs.org on November 21, 2018

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ACS Applied Nano Materials

Epitaxial Yttria-Stabilized Zirconia on Muscovite for Flexible Transparent Ionic Conductors Ping-Chun Wu1, Yu-Ping Lin1, Yung-Hsiang Juan1, Yao-Ming Wang2, Thi-Hien Do1, HorngYi Chang3 and Ying-Hao Chu1,4,5* Affiliations 1

Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan

2

Industrial Upgrading Service Department, Metal Industries Research & Development Centre, Kaohsiung 81160,

Taiwan 3

Department of Marine Engineering, National Taiwan Ocean University, Keelung 20224, Taiwan

4

Center for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu 30010, Taiwan

5 Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan

*Corresponding to [email protected]

Abstract The advantages of ionic conductors have attracted great interests worldwide since they can fit the requirements which standard electrical conductors struggle to meet. Yttria-stabilized zirconia (YSZ) is the most common ionic conductors for various practical applications. In this study, in order to bring ionic conductors into the field of soft technology, transparent YSZ films with superior mechanical flexibility were epitaxially grown on muscovite substrate by pulsed laser deposition. The epitaxial relation between YSZ and muscovite has been well established, indicating a high crystallinity thin film. The heterostructure of YSZ/muscovite exhibits excellent ionic conductivity with great mechanical flexibility. The smallest bending radius of this heterostructure can be achieved is ~ 10 mm with excellent mechanical cyclabilty (>800 cycles) and stability (>105 s), serving as a new platform to fabricate highly flexible ionic conductors.

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Introduction Ionic conductors are highly demanded since they can meet the requirement in the development of environmental-friendly devices. Nowadays, the climate change has been a serious problem impacting our daily life. There are many issues waiting to be solved, including global warming, air pollution and nuclear hazard. For example, the monitoring of environment has become essential for effective emission control. In the monitoring of greenhouse gases, ionic conductors are required and widely used since they are sensitive to CO and NOx gases.1,2 Besides, several solutions are proposed to pursue clean energy technology, including the improvement of energy efficiency, the reduction in the petroleum consumption, and the supply of eco-friendly energy. Solid oxide fuel cell (SOFC)3,4 is one of the demanding solutions with desirable advantages, including high efficiency, contamination-free and long life cycle. Ionic conductors are required in the development of SOFC as electrolytes.5 In addition, ionic conductors are also needed in many fields such as membranes,6 batteries7 and electrochromic windows.8 In the selection of ionic conductors, oxide ionic conductors are widely adopted since they can be easily miniaturized, contributing an increase in ionic conductivity,9 and can be operated at elevated temperatures.10 Yttria-stabilized zirconia (YSZ) is the most common used oxide ionic conductor in the form of film.11,12 YSZ is corrosion resistant and chemically stable at high temperatures with low thermal conductivity.13,14 Since YSZ is usually utilized at high temperature, the change in temperature will cause a change in sample length. Due to the different thermal expansion coefficients of YSZ and most commercial substrates, when heating to high temperature, the change in length will also be different between YSZ and substrates, which will cause a large mismatch, forming defects and leading to sample cracking.15 Actually, the thermal expansion issue of YSZ still remains unsolved.16 Many researchers have paid their efforts, such as changing the doping level of YSZ or looking for a suitable substrate, to reduce the impact caused by thermal expansion mismatch.17-19 Recently, there are many reports on the establishment of functional oxides on layered muscovite via van der Waals epitaxy.20,21 One critical feature of such an approach is the weak interaction between the films and muscovite substrate due to the nature of van der Waals epitaxy. In addition, since they are all oxides, they have close thermal expansion coefficient, potentially solving the problem of thermal expansion mismatch. Additionally, functional oxides on muscovite are reported flexible, retaining the pristine properties, creating a field of 2 ACS Paragon Plus Environment

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using inorganic materials for flexible electronics.22,23 In fact, flexible ionic conductors have been eagerly expected to meet emerging technological demands that electronic conductors cannot achieve. For instance, Shi et al.24 demonstrated touch sensors based on ionic conducting elastomers. Furthermore, Sun et al.25 used ionic conductors to demonstrate a sensory sheet which is highly stretchable, transparent, and biocompatible. The highly sensitive feature makes it a good candidate to wearable devices. Kim et al.26 also demonstrated a highly stretchable and transparent ionic touch panel which shows fast-sensing and can be operated under >1000% strain. It should be noted that the aforementioned reports are based on polymers, flexible inorganic ion conductor film remains elusive. Moreover, recent studies made on YSZ have shown that the ionic conductivity can be influenced by the grain boundaries.27 Indeed, large number of grain boundaries lead to an increase of ionic resistivity in YSZ films. As a result, highly dense or epitaxial YSZ films are needed to eliminate the influence of grain boundaries. Thus, in this study, we chose muscovite mica as the substrate to fabricate epitaxial 8 mol% YSZ films directly by pulsed laser deposition (PLD). The heteroepitaxy of YSZ/muscovite was examined by a combination of various diffraction techniques. The ionic conductivity and activation energy were characterized by AC impedance measurement at various temperatures. Further, to take the flexible advantages of muscovite, the YSZ/muscovite heterostructure has been tested under various bending conditions, showing superior mechanical flexibility. This work advances YSZ-based ionic conductors into the realm of soft technology. Results & Discussions The samples were fabricated by PLD equipped with an in-situ reflective high-energy electron diffraction (RHEED) to monitor the growth condition in real time, as shown in Figure 1 (a). The sharp RHEED streaks and the presence of Kikuchi lines indicate the flat and wellcrystalline surface structure of muscovite. Here, taking along [100] and [010] of muscovite, two set of streaks can be identified, as shown in the lower panel in Figure 1 (b) and (c), respectively. As soon as the growth started, the streaks immediately disappeared and a new set of streaks appeared after the phase of YSZ had been deposited. According to the simulation on the new set of streaks, we can generalize that YSZ [112], upper panel in Figure 1 (c), parallels to muscovite [100] and YSZ [110], upper panel in Figure 1 (d), parallels to muscovite [010], respectively. This result suggests the epitaxial relation between YSZ film and muscovite substrate. Thus, the out-of-plane relation can be determined as YSZ [110] parallel to muscovite [001]. Furthermore, the corresponding atomic structures of surface and simulated reciprocal



lattices along YSZ [111] and muscovite [001] zone axes are compared as shown in Figure 1 (d) and (e), respectively.

Figure 1. Schematics and RHEED patterns (a) Schematic of YSZ thin film grown on flexible muscovite by pulsed laser deposition. Upper panels in (b) and (c) show RHEED patterns of YSZ taken along its [112] and (c) [110], respectively; The lower panels in (c) and (d) are the corresponding RHEED patterns of muscovite before growth, taken along its [100] and [010], respectively; (d) The surface structure of muscovite (001) in real space (right) and reciprocal space (left); (f) The surface structure of YSZ (111) in real space (right) and reciprocal space (left).

To further investigate the structural information, x-ray diffraction (XRD) was employed. From the XRD normal scan shown in Figure 2 (a) reveals (111)-oriented YSZ film on (001) muscovite substrate. There is no secondary phase detected, indicating the feature of pure YSZ phase on muscovite. The calculated d-spacing of YSZ (111) extracted from the XRD normal scan is ~3.04 Å, which is larger compared to the bulk value (2.99 Å).28 Such a result is attributed to the growth condition at low oxygen pressure (~10-6 torr). However, the film became polycrystalline at high oxygen pressure (>10-5 torr). Besides, the crystallinity is revealed by the full width at half-maximum (FWHM) (~3.7 o) of the rocking curve around YSZ (111) as shown in Figure 2 (b). Moreover, the phi-scan was employed to build up the epitaxial 4 ACS Paragon Plus Environment

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relation between YSZ and muscovite, as shown in Figure 2 (c) and (d). The reflection of YSZ (220) can be detected every 60o, indicating the multi-domain feature, since (111)-oriented YSZ thin films exhibits a 3-fold symmetry. The good alignment between the peaks of muscovite (024) and YSZ (220) at every 120o interval confirms the in-plane epitaxial relation as YSZ [220]//muscovite [010], which is consistent with the results of RHEED.

Figure 2. (a) X-ray normal scan of the YSZ/muscovite heteroepitaxy; (b) The Rocking curve of YSZ (111); (c) The phi scan of YSZ {220} and muscovite {024}.

In order to examine the quality of the interface, high-resolution transmission electron microscopy (HR-TEM) was introduced. The cross-sectional HR-TEM image represented in Figure 3 (a) shows a defect-free and sharp interface of the YSZ/muscovite heteroepitaxy. The corresponding fast Fourier transform (FFT) patterns of YSZ and muscovite are shown in Figure 3 (b) and (c), respectively, revealing the epitaxial relation as YSZ [112]//muscovite [100], consistent with the XRD and RHEED results. Based on the results mentioned above, we can confirm a high quality of (111)-oriented YSZ thin films with the correct symmetry have been epitaxially grown on muscovite substrate. 5 ACS Paragon Plus Environment


Figure 3. (a) The cross-sectional HR-TEM of YSZ grown on muscovite substrate; The corresponding fast Fourier transform patterns of (b) YSZ and (c) muscovite, respectively.

Since the epitaxial relation has been well established, we now pay attention to the ionic conductivity of YSZ/muscovite. The thickness of YSZ thin films adopted is 50 nm. A thickness effect of YSZ on ionic conductivity has been discussed in page 2 of Supporting Information. The AC impedance of YSZ/muscovite at various temperatures are illustrated in Figure 4 (a). The ionic conductivity of YSZ is dominated by oxygen vacancies. These vacancies are mobile at high temperatures and give rise to high oxygen ionic conductivity. As shown in Figure 4 (a), the impedance became smaller with the increase of temperature, indicating a higher conductivity at high temperature. We use ZView software to analyze the resistance of YSZ and calculate the conductivity (0.8 S/cm at 700 oC). Besides, we also conducted the impedance of YSZ/muscovite under bending condition. The result under a bending radius of 12 mm is illustrated in Figure 4 (b). After the bending, the impedance of YSZ/muscovite, released from the bending mold, was measured again in the flat condition, as shown in Figure 4 (c). From these results, the behavior remained similar after bending and releasing. Further, the variation of ionic conductivity with temperature supports the thermally activated process and obeys the Arrhenius equation (eq 1).

𝜎=

𝐴 𝑇𝑒

― 𝐸𝑎 𝑘𝑇

(1)


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Where 𝜎, A, Ea, k and T denote the conductivity, material constant, activation energy, Boltzmann constant and temperature in Kelvin, respectively. By taking the logarithm of the ionic conductivity versus the inverse of the temperature, we can plot Figure 4 (d) and the activation energy can be calculated. There are two mechanism in YSZ/muscovite, resulting two activation energies in the high and low temperature ranges. In the low temperature range, the activation energy is the contribution from bulk YSZ, while in the high temperature range is the contribution from both bulk and interface.29 We have calculated the activation energies by taking 550 oC as the transition as shown in Figure 4 (d). Moreover, the conductivity of YSZ/muscovite is higher than that of bulk. The reason was suggested by Fabbri30 that, in the form of thin films, crystalline symmetry is broken at interfaces, resulting in a local space charge region due to the redistribution of ionic defects. The high ionic conductivity at the interface regions is attributed to the high concentration of mobile point defects, resulting higher ionic conductivity in thin films than bulk. As a result, the ionic conductivity of YSZ/muscovite is higher than bulk because the contribution from the surface conduction and heteroepitaxial stress becomes more relevant. To sum up, under these three kinds of bending conditions, the impedance behavior of YSZ/muscovite remained without any decay, indicating the bendable and recoverable property.



Figure 4. Impedance diagram of the YSZ/muscovite heterostructure (a) flat, (b) under 12 mm bending radius and (c) release from 12 mm bending radius at 400 oC. The insets in (a), (b) and (c) are impedance diagram at higher temperature (d) ionic conductivity versus the reciprocal temperature. The bendable test of YSZ/muscovite is an important part to show mechanical flexibility. To confirm the stability of this system, YSZ/muscovite was measured under two different bending conditions, the flexi-in mode, YSZ/muscovite is under compressive strain and the flexi-out mode, YSZ/muscovite is under tensile strain. These modes are described in the lower inset of Figure 5 (b). Figure 5 (a) shows the Arrhenius plot of various bending conditions with almost identical feature. The calculated activation energies are depicted in Figure 5 (b). The ionic conductivity and activation energy showed negligible change while the YSZ/muscovite heterostructure was exposed under either compressive or tensile strain, suggesting the YSZ/muscovite retains its original property under bending conditions. Last, more bending tests were adopted to build up the mechanical cyclability and the duration of YSZ/muscovite heterostructure. The activation energies as a function of bending cycles are displayed in Figure 5 (c). The YSZ/muscovite was bonded on a homemade bending stage with the bending radius of 10 mm. After 800 bending cycles, the activation energies and ionic conductivities, as depicted in the inset of Figure 5 (c), show a variation less than 10%, indicating the cyclability of YSZ/muscovite. The duration test of YSZ/muscovite was performed by bending the heterostructure to the bending radius of 10 mm and measured the impedance as a function of time. The ionic conductivity remained the value at 0.6 S/cm for up to 105 seconds, confirming the good duration of the YSZ/muscovite heterostructure. Through the tests mentioned above, the YSZ/muscovite retains the original properties of YSZ and can be brought to the playground for soft technology.


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Figure 5. (a) The ionic conductivity versus reciprocal temperature of YSZ/muscovite, including three kinds of bending conditions, flat, flexi-in and flexi-out; (b) The activation energy extracted from (a) with different bending radius of compressive and tensile stress; (c) The cyclability test and (d) duration test of the YSZ/muscovite heterostructure under the bending radius of 10 mm at 700o C. Conclusion In conclusion, we have fabricated the epitaxial YSZ films on muscovite substrate with high crystallinity. The epitaxial relation has been well established by RHEED, XRD and HRTEM. By measuring the impedance from 400 to 700 oC, the ionic conductivity can be calculated. Further, the Arrhenius equation was adopted to calculate the activation energy, which is close to the one of bulk, confirming the heterostructure retains the inherent property. Besides, the mechanical flexibility, cyclability and duration of the heterostructure was demonstrated through the bending tests. Furthermore, the YSZ/muscovite heterostructure possesses high optical transmittance over 90 % in the range of visible light (380 nm to 800 nm), as shown in Figure S2 of the supporting materials. The high optical transmittance feature 9 ACS Paragon Plus Environment


has added the opportunity for YSZ/muscovite as smart windows and touch panels. A preliminary demonstration based on YSZ/muscovite as a gas sensor is shown in Figure S3 of the supporting materials. This work has advanced the fabrication of YSZ films on flexible substrate, bringing the ionic conductor into the realm of soft technology. Experimental sections Sample preparation. The chemical formula of muscovite is KAl2AlSi3O10(OH)2. The muscovite we used in this study is the natural one purchased from The Nilaco Corporation. The dimension of the substrates is 1cm*1cm and, to become flexible, the thickness of muscovite should be thinner than 50 μm. YSZ films were grown on muscovite using pulsed laser deposition with an in-situ RHEED. A KrF excimer laser ((λ =248 nm) at a repetition rate of 10 Hz and the fluence of ∼1.75 J cm−2 was used to ablate the stoichiometric YSZ targets with 99.99 % purity purchased from Ultimate Material Technology Corporation. During the growth, the temperature of substrate was kept at 650 oC in an oxygen pressure of 1 × 10 ―6 Torr. RHEED was adopted to monitor the growth condition and growth rate in real-time. Structural analysis. The crystal structure and epitaxial relation were established by synchrotron based X-ray diffraction at beamline 17A in the National Synchrotron Radiation Research Center, Taiwan. Cross-sectional TEM specimen was prepared by focused ion beam (FIB) technique (FEI Nova 200). TEM specimen was then examined in the JEOL JEM ARM 200F microscope. Electrochemical Impedance Spectroscopy. The measurements of electrochemical impedance were carried out by Zahner Zennium electrochemical workstation with a frequency range up to 4 MHz and a maximum current up to 2.5 A. The samples were put in a heating furnace and two-probe measurement was adopted. Bending tests. A computer aided homemade bending stage was used for the bending tests. When the sample was put on the stage, one side of the sample was fixed, the other side was movable to create the suitable bending radius of the sample. Acknowledgement We thank the support from Ministry of Science and Technology, Taiwan (Grant Nos. MOST 106-2119-M-009-011-MY3, 106-2628-E-009-001-MY2, 106-2923-M-009-003-MY2, 106-2218-E-009-021) and The SPROUT project of Ministry of Education, Taiwan. This work 10 ACS Paragon Plus Environment

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is financially supported by the Center for Emergent Functional Matter Science of National Chiao Tung University from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan.

Supporting Information Available A thickness effect of YSZ on ionic conductivity has been discussed. Further, YSZ/muscovite heterostructure possesses high optical transmittance over 90 % in the range of visible light (380 nm to 800 nm), as shown in Figure S2. Last, a preliminary demonstration based on YSZ/muscovite as a gas sensor is shown in Figure S3.

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