Emergence of Rapid Oxygen Surface Exchange Kinetics During In

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Emergence of Rapid Oxygen Surface Exchange Kinetics During In Situ Crystallization of Mixed Conducting Thin Film Oxides Ting Chen, George Harrington, Juveria Masood, Kazunari Sasaki, and Nicola H Perry ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21285 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

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ACS Applied Materials & Interfaces

Emergence of Rapid Oxygen Surface Exchange Kinetics During In Situ Crystallization of Mixed Conducting Thin Film Oxides Ting Chen(a,b), George F. Harrington(b,c,d,e), Juveria Masood(f), Kazunari Sasaki(a,b,c,d), and Nicola H. Perry(b,g)* (a) Kyushu

University, Department of Hydrogen Energy Systems, 744 Motooka, Nishi-ku Fukuoka 819-0395, Japan (b) Kyushu University, International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), 744 Motooka, Nishi-ku Fukuoka 819-0395, Japan (c) Kyushu University, Next-Generation Fuel Cell Research Center (NEXT-FC), 744 Motooka, Nishi-ku Fukuoka 819-0395, Japan (d) Kyushu University, Center for Co-Evolutional Social Systems, 744 Motooka, Nishi-ku Fukuoka 819-0395, Japan (e) Massachusetts Institute of Technology, Department of Materials Science and Engineering, 77 Massachusetts Ave., Cambridge, MA 02139, U.S.A. (f) Northwestern University, Department of Materials Science and Engineering, 2220 Campus Drive, Evanston, IL 60208, U.S.A. (g) University of Illinois at Urbana-Champaign, Department of Materials Science and Engineering, 1304 W Green St, Urbana, IL 61801, U.S.A.

*E-mail of the Corresponding Author: [email protected] Key words: Amorphous, In situ Crystallization, Oxygen surface exchange, Oxygen electrodes, Optical absorption, Thin films Abstract The oxygen surface exchange kinetics of mixed ionic and electronic conducting oxides (MIECs) play a critical role in the efficiency of intermediate-to-high temperature electrochemical devices. While there is increasing interest in low temperature preparation of MIEC thin films, the impact of the resultant varied degrees of crystallinity on the surface exchange kinetics has not been widely investigated. Here we probe the effect of crystallization on oxygen surface exchange kinetics in situ, by applying an optical transmission relaxation (OTR) approach during annealing of amorphous films. OTR enables contact-free, in situ, and continuous quantification of the oxygen surface exchange coefficient (kchem); we previously applied it to PrxCe1-xO2-δ and SrTi1-xFexO3-δ thin films. In this work, the OTR approach was successfully extended to other mixed conducting thin film compositions for the first time (i.e., perovskite SrTi0.65Co0.35O3-δ and Ruddlesden-Popper Sr2Ti0.65Fe0.35O4±δ), as well as to Pr0.1Ce0.9O2-δ, enabling quantification of the kchem of their native surfaces and comparison of the behavior of films with different final crystal structures. All the thin films were prepared by pulsed laser deposition at 25 ºC or 700-800 ºC and subject to subsequent ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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thermal treatments with simultaneous OTR monitoring of kchem. The surface roughness, grain size, and crystallinity were evaluated by scanning probe microscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Fluorite Pr0.1Ce0.9O2-δ films grown at 25 ºC did not exhibit an increase in kchem after annealing, as they were already crystalline as-grown at 25 ºC. For all other compositions, OTR enabled in situ observation of both the crystallization process and the emergence of rapid surface exchange kinetics immediately upon crystallization. Perovskite SrTi0.65Co0.35O3-δ and Ruddlesden-Popper Sr2Ti0.65Fe0.35O4±δ thin films grown at 25 ºC exhibited at least 1-2 orders of magnitude enhanced kchem after annealing compared to highly crystalline thin films grown at 800 ºC, indicating the benefits of in situ crystallization. 1. Introduction The oxygen surface exchange coefficient (kchem) is a key metric impacting the efficiency of mixed ionic and electronic conductors (MIECs) in various electrochemical devices, such as gas separation membranes 1, solar thermo-chemical cells 2, oxygen sensors 3, and reversible solid oxide cells (RSOCs)

4-5.

Particularly, for low-to-intermediate temperature R-SOCs that are of interest for longer

lifetimes, the dominant contribution of oxygen electrodes, compared to the electrolyte or fuel electrodes, to large overpotentials and efficiency losses has been reported by a number of groups 6-7; this limitation is mainly caused by the slow kinetics of oxygen exchange

8-9.

Therefore,

understanding and enhancing the oxygen surface exchange kinetics of mixed conductors is a necessary requirement for improving the electrochemical performance. The pulsed laser deposition (PLD) technique is advantageous for the study of oxygen surface exchange kinetics given its ability to prepare high quality complex oxide thin films, with welldefined geometry and known active surface area 10-12. In this work, PLD was used to prepare model thin film electrodes with nanoscale thickness and controllably varied microstructure and crystallinity. Recently, amorphous thin films have attracted attention because of their advantages of uniformity and low fabrication temperature, which enable predictable behavior and low energy processing on thermally-sensitive substrates and devices

13-14.

Such amorphous films have

applications in, e.g., electronics as thin-film transistors 15-16, memristors 17, and dielectrics 18-19, and in photovoltaics as electron and hole transport layers 20-21. The widespread potential applications of amorphous and nanocrystalline thin films, combined with the need to often process these film components within a limited thermal budget, have also driven interest in their crystallization behavior

22.

For example, Heiroth et al. have reported that crystallization temperatures can be

ACS Paragon Plus Environment

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decreased by annealing amorphous thin films, compared to directly depositing highly crystalline thin films by PLD 13. Crystallization studies of piezoelectric lead zirconate titanate (PZT) thin films 23-25,

and ionically conducting yttria-stabilized zirconia (YSZ)

13, 26

or ceria-based thin films

27-30,

have focused on the crystallization kinetics and thermally-driven microstructural evolution, including microstrain and grain growth kinetics and stagnation 31. However, there are very few crystallization studies focusing on oxygen electrode materials or more generally on MIECs. Low temperature processing and/or operation of MIECs is additionally of interest from the perspective of avoiding degradation associated with high temperature cation diffusion and surface segregation 5, 32-33. Ryll et al. reported an electronically conducting amorphous bismuth ruthenate thin film with a high electrical conductivity of 7.7 ×104 S m-1 at room temperature. The electrical conductivity initially increased during heating to 409 ºC, followed by a decrease during heating to 570 ºC, while it crystallized (into Bi3Ru3O11 or Bi2Ru2O7) due to the chemical or structural changes, and concurrent disintegration, respectively

34.

Amorphous

lanthanum nickelate (LaNiO3-δ and La4Ni3O10-δ) thin films prepared by spray pyrolysis were also tailored by heat treatments and showed high electrical conductivity after crystallization, suggesting promise as a potential solid oxide fuel cell cathode

35.

A partially crystalline La0.6Sr0.4CoO3-δ

cathode was reported by Evans et al., within a micro-fuel cell that showed a power density of 200 mW cm-2 at 400 ºC, which indicated the potential of high performance oxygen electrodes fabricated at lower temperatures 36. Cavallaro et al. also recently demonstrated the general applicability of thin film

amorphous/poorly-crystalline

cathodes

(e.g.,

La0.8Sr0.2CoO3-δ,

La0.6Sr0.4CoO3-δ,

La0.5Sr0.5Mn0.5Co0.5O3-δ) for low temperature SOFCs, which showed comparable performance at 350-400 ºC to the highly crystalline films

14.

Their amorphous-grown films exhibited, at 400 °C

after a pre-anneal, higher tracer oxygen diffusivity (D*), slightly higher tracer oxygen surface exchange coefficient (k*), and lower electronic conductivity than the highly crystalline counterparts. A limitation of the tracer exchange approach is that there is no means to continuously monitor k* in situ, e.g., during microstructural/crystallinity changes. With the exception of the data in this last very interesting report, comparisons of surface exchange kinetics for amorphous vs. crystalline MIECs are lacking. Further, no studies (outside of our work) have directly monitored in situ the impact of crystallization and associated microstructure evolution on the oxygen surface exchange kinetics, a critical metric of mixed conductor performance. Therefore, a greater understanding of the influence of crystallization on oxygen surface exchange behavior is needed.



In our previous study

32,

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amorphous SrTi0.65Fe0.35O3-δ (STF35) thin films were not able to exhibit

optically measurable oxygen exchange, while excellent crystallinity, obtained at high growth temperatures, coexisted with Sr segregation and therefore non-optimal kchem. However, crystallization at intermediate temperatures was used to obtain much faster surface exchange kinetics. We found that the in situ crystallization process could contribute to much faster surface oxygen exchange kinetics for the perovskite STF35 thin films possibly due to the generation of exposed pristine surfaces, nanosized gas channels, and a degree of crystallinity 32. This finding was enabled by applying a contactless, continuous, in situ Optical Transmission Relaxation (OTR) approach to measure the chemical oxygen surface exchange coefficient (kchem). OTR permits quantification of kchem by recording the film’s optical absorption change over time, related to a change in oxygen stoichiometry, upon rapidly varying the oxygen partial pressure 37-39. In the present study, we aimed to understand how crystallization affects the oxygen surface exchange kinetics of various mixed conducting oxide compositions and structures in situ measuring kchem prior to, during, and after crystallization - and to extend the application range of OTR to different materials. We examined the crystallization-surface exchange kinetics behavior of perovskite, fluorite, and Ruddlesden-Popper structured films, and additionally explored the possibility of applying the in situ optical transmission relaxation approach more broadly, to new MIEC compositions. The perovskite SrTi0.65Co0.35O3-δ (STC35) was chosen due to the similar crystal structure and electrochemical properties to STF35

39-40,

with the expectation that it might

also show similar optical behavior for application in the OTR approach. This may be the first report of its surface exchange coefficients. For fluorite Pr0.1Ce0.9O2-δ (PCO10), monochromated 532 nm light has been used for studying the surface oxygen exchange kinetics by the OTR approach 38, 41-42, and therefore this composition is a suitable candidate for exploring the response of a fluoritestructured oxygen electrode. The Ruddlesden-Popper phase of Sr2Ti0.65Fe0.35O4±δ (RP-STF35) was selected because of our previous experience with perovskite STF35 as an optically active MIEC, although we have not seen this RP phase reported elsewhere. This paper may represent the first report of its synthesis and surface exchange kinetics. We have chosen to compare the impact of in situ crystallization on the oxygen surface exchange kinetics for mixed conductors with different crystal structures, to understand whether initial results on perovskite STF35 are representative of more general phenomena, despite expectations of different kinetics and thermodynamics of crystallization across different structures and compositions.


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In this work, a number of STC35, PCO10, and RP-STF35 thin films with different degrees of crystallinity were deposited on yttria-stabilized zirconia (YSZ) substrates at different temperatures by PLD. The microstructures and crystallinity of these mixed conducting thin films were characterized by means of X-ray diffraction, scanning probe microscopy, and transmission electron microscopy. The non-contact, continuous in situ OTR approach was employed to quantify the kchem of the native surfaces. The results demonstrate the broad applicability of OTR and can be applied to guide processing/microstructure optimization of thin film MIECs, to improve the performance of high temperature electrochemical devices, including R-SOCs. 2. Experimental approach 2.1. Thin film preparation A disk-shaped target of perovskite SrTi0.65Co0.35O3-δ (STC35) was prepared for pulsed laser deposition (PLD) by solid-state reaction plus thermal decomposition. Powders of dried SrCO3 (Wako, 99.99%), TiO2 (Wako, 99.9%) and Co3O4 (Wako, 99.9%) were homogeneously mixed and ground together (Co3O4 was decomposed from Co(NO3)3•6H2O via heating in air), uniaxially pressed at 125 MPa, and sintered in air at 1425 ºC for 6 h with heating and cooling at 2 ºC/min to obtain the >90% dense pellet. The Ruddlesden-Popper Sr2Ti0.65Fe0.35O4±δ (RP-STF35) target was also fabricated by solid state synthesis as described in the following steps. Stoichiometric SrCO3 (Wako, 99.99%), TiO2 (Wako, 99.9%) and Fe2O3 (Wako, 99.9%) powders were mixed with ethanol and added to zirconia milling beads in a polyethylene bottle and milled for 48 h. The resulting mixture was dried at 120 ºC for 12 h in an oven and then ground using an agate pestle and mortar. These powders were then uniaxially pressed and sintered in air at 1500 ºC for 6 h with a heating and cooling rate of 3 ºC/min. After that, the RP-STF35 pellet was kept under vacuum in the PLD chamber to prevent any possible structural changes and/or degradation due to atmospheric water 43. The fluorite Pr0.1Ce0.9O2-δ (PCO10) was fabricated by a modified Pechini method, and more details can be found in ref 42. Thin films with different crystalline structures were deposited from these targets by PLD at different conditions (PLD #1, Pascal Corp., Japan, and PLD #2, AOV Co., ltd., Japan). The STC35 and RP-STF35 thin films were prepared by PLD #1, and the PCO10 thin films were prepared by PLD #2. All the thin films were grown on single crystal yttria stabilized zirconia (YSZ, 13% mol Y2O3, 10 mm×10 mm×0.5 mm) substrates with both sides polished (Dalian Keri Optoelectronic Technology Corp., China, (100) direction). For PLD #1, an excimer laser beam at a wavelength of 248 nm (COMPEX Pro 50F, Coherent) was employed. The output laser energy was set at 95-100



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mJ for STC35 and 90-93 mJ for RP-STF35, both with a repetition frequency of 5 Hz and laser spot area of 2-3 mm2. The distance between the substrate and target was about 5 cm. The deposition atmosphere was maintained at 5 Pa O2 and the base pressure of the chamber approximately 4×10-6 Pa. STC35 and RP-STF35 thin films were grown at both 25 ºC and 800 ºC to obtain different microstructures and crystallinity. Two fluorite structured PCO10 thin films were grown at 25 ºC and 750 ºC, respectively. The pressure during growth was 2.5 Pa O2, the repetition rate was 5 Hz, and the laser energy was set to 250 mJ, measured directly outside the chamber as 155 mJ, and focused to an area of about 5 mm2. All the growth rates were evaluated by X-ray reflectometry (XRR) measurements on very thin calibration films grown for few minutes, with fitting by Globfit software. The growth rates for 25 ºC-grown STC35, 25 ºC-grown RP-STF35, 800 ºC-grown RPSTF35, and 25 ºC-grown PCO10 thin films were 0.007, 0.005, 0.006, and 0.079 nm per pulse respectively (as shown in ESI, Figure S1). The final thickness of the thin films used for surface exchange measurements ranged from about 40-110 nm for various STC35 thin films, about 100-150 nm for various RP-STF35 thin films, and ~300 nm for PCO10 thin films. 2.2. Characterization The light transmission spectra of STC35 thin films (700 ºC-as grown (separate growth) and 700 ºCgrown after annealing in 4% H2/ 92% N2 at 500 ºC for 5 h) were measured by ultraviolet-visible spectroscopy (V-670, Jasco Corp., Japan) in air at room temperature to identify the wavelength at which absorption changed the most upon changing the oxygen content. The surface roughness and grain size of all STC35, PCO10 and RP-STF35 thin films were characterized by scanning probe microscopy (SPM, Shimadzu Corp., Japan) in dynamic force microscopy (DFM) mode with a silicon cantilever (42 N/m, NanoWorld, Switzerland). All the RMS (root mean square) roughnesses of these thin films were calculated by the SPM analysis software. The crystallinity, orientation, and crystalline structure of all thin films were characterized by X-ray diffraction (XRD, Rigaku Corp., Japan) with coupled 2θ/ω scans by using a high resolution Ge (220)×2 monochromator with CuKα1 radiation in a 2θ range of 10-90 º with scanning speed of 0.05-1 º/min and step size of 0.010.001 º. The surface morphology, thickness, and crystallinity of 25 ºC-as grown and 25 ºC-grown after post-annealing STC35 and RP-STF35 thin films were validated through transmission electron microscopy. Thin cross-sectional lamellae were prepared using the in-situ ‘lift-out’ technique on a dual-beam focused ion beam (FIB-SEM, Helios Nanolab, 600i, FEI Company, USA). The films were first coated in Pt using a desktop sputter coater to prevent surface charging, followed by a thicker bar of Pt deposited in the FIB-SEM to protect the sample surface during thinning. The


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sections were thinned at 30 keV, followed by 5 keV and 2 keV to minimize amorphous damage to the lamellae. The sections were observed using a JEM-ARM-200F (JEOL Ltd., Japan) equipped with a Cs corrector on the imaging lens and operating at 200 keV. The valence band photoelectron spectra of Co in a STC35 thin film were recorded by X-ray photoelectron spectroscopy (XPS, ULVAC-PHI, PHI5000 Versaprobe), in which the Al K X-ray source (1486.6 eV) operated at 25.34 W with a pass energy of 58.7 eV. Angle resolved XPS measurements were performed at 20º-90º to acquire the concentration of each element (Sr 3d, Ti 2p, O 1s, and Co 2p) in the STC35 thin film surface region. Peak fitting was performed by XPSPEAK4.1 software with a “Shirley” background function. The binding energy value was referenced to the C 1s peak at 284.8 eV. 2.3. Crystallization and Optical Transmission Relaxation (OTR) measurements The oxygen surface exchange coefficient (kchem) was quantified by an in situ optical transmission relaxation (OTR) approach, which has been used for obtaining the kchem for limited mixed conducting thin film compositions, such as PrxCe1-xO2-δ (PCO) 44-45.

37-38, 41

and SrTi1-xFexO3-δ (STF)

32,

It relies upon the application of the Beer-Lambert law, where optical absorption is proportional

to the concentration of absorbing species, e.g. oxidized Pr (~Pr4+) in PCO or oxidized Fe (~Fe4+) in STF, and to the oxygen stoichiometry via electroneutrality. For example in STF, during reduction (oxygen evolution), absorbing Fe4+ is replaced by Fe3+, resulting in an increase in measured optical transmission through the STF with time. For a thin film in which the change in oxygen content over time is limited by the surface exchange kinetics, the following espresions can be used to determine kchem : 44, 46

ln I 0  ln I t  t  1  exp    ln I 0  ln I f   kchem 

L



[1] [2]

where τ, L, It, I0, and If are the time constant, thickness of thin film, the transmitted light intensities at any given time (t), initial time (0), and final time (f), respectively. More details of the defect chemistry of these materials and the OTR measurements can be found in the previous studies 32, 41. A chopped beam of monochromated and collimated light (Light source, SLA-100A, SIGMAKOKI, Co., Ltd,; Collimator, F230SMA-A, Thorlabs, USA; Chopper MC2000, Thorlabs, USA) was used



Page 8 of 35

for STC35, PCO10, and RP-STF35 studies at a wavelength of 460 nm (FL460-10 filter, Thorlabs, USA), 532 nm (FL532-10 filter, Thorlabs, USA), and 442 nm (FL441.6-10 filter, Thorlabs, USA), respectively. For in situ optical relaxation measurements of perovskite STC35 and RP-STF35 thin films (both 25 ºC-grown and 800 ºC-grown), the first set of measurements was performed during heating in 50 ºC increments from 300 ºC to 600 ºC, while changing the pO2 (N2/O2 gas mixture) at each isothermal temperature step to observe the optical relaxation at each temperature. Then the films were annealed for 1 h at 600 ºC prior to cooling and the second set of measurements. The in situ crystallization process was observed via its corresponding optical absorption change, during the measurements

32.

Then, for the second time, optical relaxation was measured at 300 ºC to 600 ºC

with the same thermal profile. Both the heating and cooling rates were about 10 ºC/min. For the PCO10 thin films (both 25 ºC-grown and 750 ºC-grown), a similar temperature profile was followed, but in this case the films were held longer at the temperatures ≤400 °C due to the longer time to reach equlibrum with changes in the pO2 and the annealing was conducted at 600 ºC for 2 h instead of 1 h. The rapid oxygen partial pressure change during the optical measurements was controlled by mass flow controllers and switches through changing the N2/O2 gas mixtures (O2/N2: ranging from 2% O2 and 100% O2, with a total gas flow rate of 100 sccm). All the gases were purified and dried by the molecular sieves and desiccants before feeding into the 11 mm- inner diameter quartz tube for OTR in the furnace. More details about the OTR instrument can be found in our previous paper 32. 3. Results 3.1. Microstructure characterization The thin films’ surface morphology was measured by scanning probe microscopy (SPM) as shown in Fig. 1. The x and y scale of each image is 1 μm ×1 μm, and the vertical dimension of the z scale is 20 nm in Figure 1(a)-(i). Fig. 1(a), (b), and (c) demonstrate the surface topography of STC35 thin films with different thermal histories: a) 25 ºC-as grown, b) 25 ºC-grown after annealing at 600 ºC for 1h, and c) 800 ºC-grown after annealing at 600 ºC for 1 h. These films exhibit root mean squared (RMS) surface roughness values of 1.48, 1.24 and 4.48 nm, respectively, showing larger apparent grain size and surface roughness with increasing growth/annealing temperature. The RMS surface roughness values of 25 ºC-as grown, 25 ºC-grown after annealing at 600 ºC, and 800 ºCgrown after annealing at 600 ºC Ruddlesden Popper-STF35 thin films were 1.93, 3.05 and 5.41 nm respectively as shown in Fig. 1(d), (e), and (f), which is consistent with the perovskite STC35 thin films and our previous perovskite STF35 results 32. The 3D topographic images of fluorite PCO10


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thin films, 25 ºC-grown, 25 ºC-grown after annealing at 600 ºC and 750 ºC-grown after annealing at 600 ºC, are shown in Fig. 1(g), (h) and (i); these exhibited low RMS surface roughnesses of 2.63, 1.08, and 0.31 nm respectively.

Figure 1. SPM-derived topography (image size: 1 μm×1 μm) of STC35 surface (a) 25 ºC-as grown, (b) 25 ºCgrown after annealing and (c) 800 ºC-grown after annealing at 600 ºC, Ruddlesden-Popper STF35 surface (d) 25 ºC-as grown, (e) 25 ºC-grown after annealing and (f) 800 ºC-grown after annealing at 600 ºC, and PCO10 (g) 25 ºC-grown , (h) 25 ºC-grown after annealing and (i) 750 ºC-grown after annealing at 600 ºC. Note: x- and yscales are all 1 μm each, while the z scale is 20 nm.

The crystallinity and crystallographic orientation were characterized by XRD. The coupled-scan patterns of STC35 thin films are shown in Fig. 2(a). A single intense peak, corresponding to the (110) reflection was observed for the 800 °C-grown STC35 film. This is the same orientation as previously observed for STF35 films grown at high temperature on YSZ, and is presumably due to the similar lattice parameters for STC35 (~3.894 Å) 40 and STF35 (~3.912 Å) 47, and hence similar lattice mismatch with the substrate. No peaks were observed from the 25 ºC-as grown STC35 film, suggesting that this film is amorphous (or possibly nanocrystalline with extremely fine crystallites); the TEM results later provide more insight into the amorphous structure of this film. For the 25 ºC grown STC35 thin film after annealing, a small (110) peak could be observed when the 2θ/ω axis was scanned with at a slow speed of 0.05 º/min, as shown in the inset of Figure 2(a).



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Fig. 2(b) shows the XRD patterns for the RP-STF35 thin film, showing again that the high growth temperature leads to high crystallinity. There is no film peak visible for RP-STF35 for the 25 ºC- as grown film, but a weak peak was detected by the slow XRD scan for the 25 ºC-grown film after annealing, therefore showing similar behavior to the perovskite STC35 thin films. Fig. 2(c) shows the coupled scan patterns for the fluorite PCO10 films grown on YSZ at different temperatures. In this case, the 25 ºC-as grown films display a peak corresponding to the (200) reflection, with minor peaks for (110) and (220) reflections, indicating that the films are at least partially crystalline, even when grown at room temperature. After annealing, the (200) reflection becomes much larger, indicating an increase in the crystallinity and preferential orientation. A clear rocking curve is obtained for the (200) reflection of the 25 ºC-grown film after annealing, indicating an increase in crystalline quality. An even larger (200) peak in the 2θ/ω scan, and narrower rocking curve is observed for the PCO films grown at 750°C, indicating good crystalline quality, as expected for highly textured PCO films grown on YSZ 48.


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Figure 2 Coupled XRD 2/ scans of (a) STC35 thin films, (b) RP-STF35 thin films and (c) PCO thin films. The inset in (a) shows the (110) reflection of the 25 °C-grown films after annealing, obtained using a slower 2/ scanning speed. The inset in (b) shows (105) reflection after the 25 °C-grown films after annealing. The inset in (c) shows the rocking curves taken from the (200) reflection of the PCO10 thin films. “*” represents the peak of sample holder. “” is the PCO (111) peak and “” is PCO (220) peak.

In order to further investigate the structure of the films, cross sections were studied using transmission electron microscopy (TEM). Figure 3 shows low-magnification bright-field images of the STC35 and RP-STF35 films deposited at ~ 25°C, both as-grown and after annealing at 600°C. Figure 3(a) shows the as-grown STC35, and Figure 4(b) shows the STC35 after annealing at 600°C. As-grown, the STC35 films are reasonably flat and continuous over the substrate, but after annealing at 600°C the surface roughness increases, and the films are no longer continuous. The same is observed for the RP-STF films. The as-grown film and film after annealing at 600°C are shown in Figure 3(c) and 3(d) respectively. A lighter region, with a different contrast in the TEM image, can be seen in Figure 3(c) along the interface between the film and the substrate. This is due to beam damage caused by the FIB during the section preparation and the electron beam during observation of the sample. There are a small number of other regions lighter in contrast for all of the films as observed in Figures 3(a-d), which are attributed to small pores in the films. (One point to note here is that the 25°C as-grown RP-STF35 thin film for TEM was deposited on a single crystal Al2O3 substrate with the same growth condition, whereas all others for TEM and all optically studied films were on YSZ. We assume that the substrate plays an insignificant role in the structure of films grown at such a low temperature, consistent with the homogeneous nucleation process as discussed later.)



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Figure 3 Bright-field TEM micrographs of (a) as-grown STC35 deposited at 25ºC, (b) STC35 grown at 25ºC and subsequently annealed at 600°C, (c) as-grown RP-STF35 deposited at 25ºC, and (d) RP-STF35 grown at 25ºC and subsequently annealed at 600°C. The red lines indicate the surface of the films and act as a guide to the eye.

Higher magnification micrographs are shown in Figure 4. As-grown films, deposited at 25 ºC, are shown in Figure 4(a) (STC35 in 4(a), and RP-STF35 in 4(c)), and films after annealing at 600°C in Figure 4(b) and 4(d) (STC35 in 4(b), and RP-STF35 in 4(d)). Fast Fourier transforms (FFTs) of a region of the film from each of the images are shown in the insets. For the as-grown films, only amorphous rings are seen in the FFTs, indicating that these films are amorphous, with no detectable crystalline structure. After heat treatments, bright spots can be observed in the FFT as expected for crystalline materials, confirming that crystallization has taken place. There is no clear preferential orientation for the crystalized films, which is consistent with the low intensity of the peaks in the XRD data in Figure 2.


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Figure 4 TEM micrographs of (a) as-grown STC35 deposited at 25ºC, (b) STC35 grown at 25ºC and subsequently annealed at 600°C, (c) as-grown RP-STF35 deposited at 25ºC, and (d) RP-STF35 grown at 25ºC and subsequently annealed at 600°C. Fast Fourier transforms from regions of the film are shown in the insets. Red dashed lines indicate estimated interface between film and Pt cover.

Figure 5(a) shows high-resolution TEM images of an as-grown STC35 film deposited at 25 ºC. No lattice fringes are observed and instead the films can be seen to have an amorphous structure. Figure 5(b) and 5(c) show STC35 and RP-STF35 films deposited at 25 ºC after annealing at 600 °C, where lattice fringes can be observed confirming that crystallization has taken place. Multiple regions with lattice fringes of different orientation can be seen, highlighting the small size of the grains.



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Figure 5 High resolution TEM images of the substrate-film interface for (a) as-grown STC35 deposited at 25 ºC, (b) 25 ºC deposited STC35 after annealing at 600°C, and (c) 25 ºC deposited RP-STF after annealing at 600°C.

3.2. Crystallization and optical studies Figure 6 shows plots of measured optical transmitted light intensity (normalized to incident intensity via chopping and lock-in-amplifiers), measuring temperature, and oxygen partial pressure as a function of time. Figures 6(a) and (b) show the response of the 25 ºC-grown STC35 thin film during the first and second measurements, respectively. (The appropriate wavelength of ~460 nm for the STC35 measurements was determined by UV-vis spectroscopy, as shown in Figure S2.) Figures 6(c) and (d) show the response of the 25 ºC-grown RP-STF35 thin film during the first and second measurements, respectively. In Figure 6(a) a very subtle, small, but fast optical response to pO2 changes appears to be visible for the STC35 film when heating the sample from 300 ºC to 500 ºC; however, it is minor compared to what we see under other conditions. By contrast, for RPSTF35, STC35, and prior perovskite STF35

32

(not shown), a large transmitted light intensity drop

was found during the heating process from 500 ºC to 550 ºC (as shown in Figure 6(a)), which we attribute to the dynamic crystallization during annealing, based on subsequent TEM studies. After this process, these films demonstrated rapid optical responses to the isothermal changes in pO2, i.e., rapid oxygen surface exchange. In order to test whether such changes were irreversible, the optical response of the films was measured again under a similar heating profile. Subsequently, after this ACS Paragon Plus Environment

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annealing up to 600 ºC and cooling, the STC35 film demonstrated very fast oxygen exchange ability at temperatures as low as 300 ºC during the second measurement (Figure 6(b)). For the 25 ºC-grown RP-STF35 film during the first measurement, there were no optical responses upon varying the pO2 below 500 ºC, so significant, measurable oxygen exchange was not observed for the amorphous film from 300 ºC to 500 ºC. After the optically apparent crystallization onset, the RPSTF35 film was able to demonstrate rapid oxygen exchange ability at 550 ºC during the first measurement and at even 300 ºC during the second measurement after cooling, as shown in Figures 6(c) and 6(d). This behavior for RP-STF35 and STC35 was also observed for the perovskite STF35 films in our previous work 32. In contrast, the optical behavior of a 25 ºC-as grown PCO10 thin film during the first measurement, which is provided in Figure S3, demonstrated obvious responses to pO2 changes even at 300 ºC, consistent with the XRD results exhibiting crystallinity from the outset. Figure S3 additionally shows that this PCO10 film does not display the sudden decrease of transmitted light intensity at ~550 ºC that is observed for 25 ºC-grown STC35, RP-STF35, and perovskite STF35 (previous work) and attributed to crystallization.



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Figure 6. In situ optical transmitted light intensity of 25 ºC-grown thin films during the annealing process (a) Asgrown STC35 during the first measurement; (b) STC35 during the second measurement (after annealing at 600 ºC then cooling to 25 ºC at the end of the first measurement); (c) As-grown RP-STF35 during the first measurement; (d) RP-STF35 during the second measurement (after annealing at 600 ºC then cooling to 25 ºC at the end of the first measurement). The light wavelength for STC35 and RP-STF35 is 460 nm and 442 nm, respectively.

Figure 7 presents an enlarged graph that compares the first obvious optical relaxation curves of 25 ºC-grown STC35 and 25 ºC-grown RP-STF35 measured at 500 ºC immediately after the crystallization to the corresponding relaxation at 500 ºC of the 800 ºC-grown highly crystalline STC35 and RP-STF35 thin films. Despite the slightly lower measuring temperature, much faster optical relaxations were observed for 25 ºC-grown thin films after in situ crystallization when changing the pO2 from 21% to 4%, compared to the highly crystalline films grown at high temperature.


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Figure 7 A comparison of the first optical relaxation of 25 ºC-grown STC35 and RP-STF35 thin films measured at 500 ºC after crystallization with 800 ºC-grown films

Figure 8 shows the oxygen surface exchange coefficient, kchem, determined from fitting the relaxations shown in Fig. 6. Here we include k values only for the pO2 step from 21% to 4% O2, when the films reduce. The plot compares the kchem of high temperature-grown vs. 25 ºC-grown thin films after annealing as a function of measurement temperature (i.e., during the second measurements shown in Fig. 6(b) and 6(d)). The red dot-dash line across each graph represents the upper limit of kchem that could be accurately measured according to the thin film thickness and gas flush time limitation of 4 s. The error bars of measured kchem in this work are given by a conservatively estimated ± 20%

32.

There are essentially two types of error to consider: the

measurement and fitting itself (relatively low error, which we estimate as well within the 20% error bars) vs. replicate studies on different films of nominally the same composition and structure (larger error, which can exceed the 20% error bars). In the former case, contributors to error are thickness variations within the films or error in thickness measurement, any error from the fitting of the data, the finite gas flush time and finite time interval of measurements for very fast relaxations, any noise in detected light intensity, and possible changes in kchem during the course of the relaxation, discussed later. In the latter type of error (replicate studies), factors such as differences in surface contamination (from handling or air during storage, etc.) or even in film stoichiometry (due to PLD deposition conditions variability) can give rise to larger variations in kchem values among different films with nominally the same compositions. Figure 8(a) compares the averaged kchem determined from the OTR approach for 25 ºC-grown STC35 thin films and an 800 ºC-grown STC35 thin film, showing that much higher kchem values were found for the films that were grown amorphous and post-annealed to crystallize. In fact, the apparent kchem values of the 25 ºC-grown STC35 after annealing are at the level of the dashed line ACS Paragon Plus Environment


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representing the upper limit of resolvable film kinetics. This overlap suggests that the optical relaxations for that film were limited by the gas flush time, and that the true kchem values are higher, given that we expect them to be thermally activated, and even at the lowest measurement temperature they overlap the upper limit for measurement. When comparing this 25 ºC-grown STC35 film to the 800 ºC-grown second measurement, which has the same post-annealing thermal history, there are at least two orders of magnitude difference in kchem, indicating the clear benefit for oxygen surface exchange kinetics for STC35 films that have undergone in situ crystallization. It should be noted that the 25 ºC-grown STC35 thin film initially demonstrated a very minor optical response to pO2 changes with an associated fast kchem value, even during the first time measurement prior to the main widespread crystallization process. This effect might be due to the presence of a small amount of nano-crystallites forming in the initial heating stage. The behavior of this perovskite STC35, with at least two orders of magnitude higher kchem after thermally crystallizing the amorphous film vs. a conventional high-temperature-grown film with similar post-annealing, is similar to that of perovskite STF35 thin films, as we published in our previous paper 32. The same benefits from in situ crystallization could also be observed for Ruddlesden-Popper STF35 thin films, as seen in Figure 8(b). The 25 ºC-grown RP-STF35 thin films after annealing showed 1-2 orders of magnitude higher kchem than the highly crystalline 800 ºC-grown thin films in the measurement temperature range of 350-500 ºC. In this figure, we think that the reason for an apparent nonthermally activated oxygen exchange above 500 ºC is that the measurements again become gas flush-time limited at this point (in other words, oxygen exchange in the films is faster than the gas flush time, so the optical response is limited by the gas flow rather than the intrinsic film properties).


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Figure 8 kchem plotted as function of measuring temperature for (a) Perovskite structured STC35 thin films, (b) Ruddlesden-Popper structured STF35 thin films and (c) Fluorite structured PCO10 thin films. Please note: second measurements were subsequently performed after an initial annealing process up to 600 ºC during the first measurements. The kchem of STC35 and RP-STF35 are averaged values from two or three thin films prepared under the same conditions.

However, for the PCO10 thin film, no obvious improvement was observed for the 25 ºC-grown film after annealing compared to the highly crystalline PCO10 thin films grown at high temperature as shown in Figure 8 (c). It should be noted that the PCO10 thin film that was grown at 25 ºC was already considerably crystalline (showing a (200) oriented peak), as demonstrated in Figure 2(c). Therefore, the annealing process for the PCO thin film did not cause the onset of in situ crystallization, nor show a positive effect on kchem. After the initial annealing process (first measurement), both PCO thin films showed worse kchem during the second measurement, possibly because of the extrinsic Si surface-poisoning-induced degradation 42.



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To the authors’ knowledge, this study is the first time that the OTR technique has been applied to thin films with the STC35 and RP-STF35 compositions. Clear optical responses to pO2 changes for crystalline films were observed, but in order to make sure that the STC35 and RP-STF35 thin films have optical relaxations that are in the linear regime 37, 49-50, and can accurately be fit to one k value per relaxation, the in situ optical relaxation measurements were performed over different sizes of pO2 steps for these two thin films. It is important to verify that the pO2 steps used are small enough that the changes in point defect chemistry in the films with pO2 are not contributing to significant changes in k over the course of the relaxation. If k is approximately constant over the pO2 step size chosen, then the fitted values are more robust. The optical relaxations resulting from different step sizes of oxygen pressure change (both for oxygen release: 21%O2→4% O2, 100% O2→4% O2, 80% O2 →4% O2, 50% O2 →4% O2, 10% O2 →4% O2; and oxygen incorporation: 4% O2 →21% O2, 2% O2→21% O2, 10% O2→21% O2) are shown in Fig. 9(a) and (b). The optical measurements for this part of the study were performed at 600 ºC and 450 ºC for STC35 and RP-STF35 800 ºC-grown thin films, respectively, and the measurement temperatures and films’ thermal history were chosen to ensure that the films’ intrinsic relaxation times were sufficiently long compared to the gas flush time. These measurements were performed after the first and second optical measurements that were already discussed. The fitted kchem values for STC35 and RP-STF35 from these different pO2 step sizes are listed in Table 1. For the STC35 thin film, the similar k values obtained across this range of conditions, with a standard deviation of ~8% for a given final pO2 value, suggest that the 4% O2 → 21% O2 and 21% O2 → 4% O2 steps used in the present work are not causing significant deviations in k value compared to using a smaller pO2 step size. For the RP-STF35 thin film used in this last part of the study, most relaxations fit to one k value, as in the previous results in this paper; however, two k values (k1 and k2) were determined from the first two relaxation curves in this last measurement; k1 was much higher than k2. Two k values may originate from inhomogeneities in the structure/composition/surface chemistry of the films, and have been observed by other groups for other compositions 49. We discuss the structural inhomogeneities observed via the RP-STF35 Xray diffraction patterns in sections 4.1 and 4.4, which may relate to this behavior. Additionally, given the low symmetry of the RP-STF35 structure, its surface exchange behavior is likely to be significantly anisotropic. Therefore, two k values could be caused by the presence of surface facets with different orientations. After these first two relaxations with two k values, all the RP-STF35 relaxations evinced more typical relaxation behavior with 1 associated k value in Table 1, suggesting that the film’s structure/chemistry evolved over time. Relevant to this part of the study, the k values for the RP-STF35 film also remained relatively unchanged by different sizes of pO2 steps, with no systematic dependence on pO2 step size or initial pO2, again indicating that the pO2 ACS Paragon Plus Environment

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step size used in the present work was appropriate. As has been observed in other studies 49, 51-53, for both compositions the k values upon oxidation appear to be, on average, slightly faster than those upon reduction. Taken together, these results demonstrate the precision and feasibility for the in situ OTR approach for these new perovskite STC35 and Ruddlesden-Popper STF35 thin film compositions.



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Fig. 9 The in situ optical relaxation at different step sizes of pO2 for (a) 800 ºC-grown STC35 thin film measured at 600 ºC and (b) 800 ºC-grown RP-STF35 thin film measured at 450 ºC, with the representative relaxation fitting of (c) 800 ºC-grown STC35 thin film upon a reduction process of 21 % O2 to 4% O2 measured at 400 ºC; (d) 800 ºC-grown RP-STF35 thin film upon a reduction process of 21 % O2 to 4% O2 measured at 450 ºC.

Table 1 The quantified kchem with different pO2 steps by OTR approach for STC35 and RP-STF35 thin films. The uncertainty is conservatively estimated as ±20%.

pO2 steps

kchem of STC35 measured at 600 ºC

kchem RP-STF35 measured at 450 ºC

/(10-7cm s-1)

/(10-7cm s-1) (k1 & k2)

21%O2→4% O2

6.25 ±1.25

4.77 ±0.95

0.08 ±0.02

100% O2→4% O2

3.21 ±0.64

4.47 ±0.89

0.15 ±0.03

80% O2→4% O2

2.79 ±0.56

4.38 ±0.87

Null

50% O2→4% O2

2.68 ±0.54

5.12 ±1.02

Null

10% O2→4% O2

2.61 ±0.52

5.01 ±1

Null

4% O2→21% O2

4.90 ±0.98

10.25 ±2.1

Null

2% O2→21% O2

4.95 ±0.99

7.54 ±1.51

Null

10% O2→21% O2

4.85 ±0.97

7.18 ±1.44

Null

4. Discussion 4.1 Microstructure vs. Growth Conditions All high temperature-grown thin films demonstrated high crystallinity with a single out-of-plane orientation for each phase. However, the RP-STF35 film exhibited additional structural changes after being deposited at high temperature. In Figure 2(b), it can be seen that the 800 °C-grown RPACS Paragon Plus Environment

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STF35 film exhibits apparently split peaks, which appear to merge after annealing. The split peaks indicate structural, and therefore likely compositional, inhomogeneity in that film. By refining the RP-STF35 bulk pellet diffraction pattern, and comparing the films’ patterns, we suggest that the peaks initially correspond to the (103) reflection of the 214 (n=1) RP phase with a shoulder of the (105) reflection of the 327 (n=2) RP phase. After annealing, the peak position is somewhat intermediate but may align most closely with the (105) reflection of the 327 (n=2) phase. In other work, structural changes of Ruddlesden-Popper phase films have been observed, including dynamic layer rearrangement of B-site cations within the Ruddlesden-Popper phases (as has been seen for Ti-O or Ni-O layers in Sr2TiO4 or La3Ni2O7) during heat treatment 54, or the intercalation of, e.g., atmospheric water into the Sr-O layers accompanied by a slight c-axis expansion 43. Regardless of the exact cause, this change from split peaks to merged peaks might indicate a transition from a less homogeneous to more homogeneous structure via annealing, aided by cation diffusion. Perovskite and Ruddlesden-Popper structured films grown at 25 ºC were amorphous according to XRD and TEM. On the other hand, in the fluorite structured PCO10 XRD patterns (Figure 2(c)), a measurable (200) reflection peak was found for films deposited even at 25 ºC, as well as weaker peaks corresponding to the (111) and (220) reflections. Initially we hypothesized that the higher laser energy used to grow the PCO10 films could have caused this structural difference vs. the perovskite films; however, subsequent PCO10 films grown at comparable energies to the perovskite and RP films at 25 ºC also evinced crystallinity. These results therefore suggest a considerably lower intrinsic crystallization temperature for fluorite structured films compared to the other perovskite and Ruddlesden-Popper structured films 35, 55. In prior work 32 we showed that perovskite STF35 films need to be grown at temperatures in excess of 550 °C in order to demonstrate any crystallinity, as-grown, by XRD. The atomic-scale origins of the thermodynamic and kinetic reasons for such a large temperature difference to form these various crystal structures/compositions are not well explored, to the authors’ knowledge. Films are grown at low temperature in a nonequilibrium state with kinetic limitations to ion diffusion that are eventually thermally overcome to nucleate and grow the more energetically stable crystalline phase 22. From a kinetic perspective, for the fluorite phase, perhaps the presence of only one cation sublattice with a single coordination environment lowers the average diffusion distance needed to convert from an amorphous to crystalline structure. For the perovskite and Ruddlesden-Popper structures, the multiple cation sites and species involved could lead to higher kinetic barriers/longer diffusion distances to reaching correct crystalline ion arrangements. Another possible contribution is the thermodynamic driving force for crystallization, which is the Gibbs free energy difference between the amorphous and



crystalline phases (considering also any interfacial contributions)

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22.

Perhaps that driving force is

significantly larger for fluorite-structured PCO10; this hypothesis would be consistent with the higher melting temperature of CeO2

56

compared to that of SrTiO3

57

and the typically lower

Madelung energies of fluorites vs. those of perovskites 58. We note that the PCO10 film behaved differently to all others (perovskite STF35 in prior work, RPSTF35, STC35) in terms of how its surface topography evolved with thermal treatments. PCO10 became increasingly smooth with increasing growth temperature, while other films increased in roughness with increasing growth temperature or annealing temperature. Possible differences in film-gas and film-substrate interfacial energies may help to explain why the 750 ºC-grown PCO10 thin film had smaller roughness compared to the other thin films 48. Perovskite and Ruddlesden-Popper films that were annealed up to 600 ºC after deposition at 25 ºC demonstrated a nanocrystalline structure with randomly-oriented crystallites. In Figures 5(b) and 5(c), the observed lattice fringes in some sections of the high-resolution TEM images indicate the partially crystalline state of 25 ºC-grown STC35 and RP-STF35 after annealing, which is also consistent with the XRD results, showing low intensity film peaks and poor crystalline quality, as in Figures 2(a)-(b). The presence of varied orientations of these crystallites, and no preferential orientation in the fast Fourier transforms in Figure 4, suggests that the nucleation process is somewhat homogeneous within the film. If crystallites nucleate only heterogeneously, at the interface of the film with the substrate, one might expect more oriented grains to grow and an observable texture to develop in the films, as is the case with the films grown at high temperatures. 4.2 kchem vs. Crystallization This is the first study investigating how in situ crystallization affects the oxygen surface exchange kinetics of thin films with a variety of crystal structures by an in situ optical transmission relaxation approach. The results on perovskite STC35, Ruddlesden-Popper phase STF35, and also our previous study of perovskite STF35 32 all demonstrated the benefit to oxygen exchange kinetics of a dynamic crystallization approach applied to amorphous thin films. The 25 ºC-grown amorphous STC35 and RP-STF35 thin films did not appear to exhibit significant oxygen exchange ability optically under 500 ºC, as shown in Figure 6(a) and 6(c), which is attributed to the low crystallinity, and possibly due to corresponding low oxygen or electron mobility in the lattice

59

and low

conductivity 60-61, although measurements of these properties on the films in this study remain tasks


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for future work. That said, we were able to observe some very minor but fast optical transmission changes for the 25 ºC-grown STC35 thin film when the pO2 changed in the 300-500 ºC range, which might be attributed to formation of a small volume of nano-crystallites during the initial heating, enabling oxygen exchange in that small region. The time constant of those minor relaxations was near the gas flush time, suggesting that it cannot be converted into an intrinsic film kchem value, but that the associated kchem is at least as fast as the dashed line in Figure 6(a). From 500-550 ºC, the STC35 and RP-STF35 films were starting to significantly crystallize during the annealing, accompanied by the prompt drop of transmitted light intensities, which we attribute primarily to the oxidation process of Co3+ → Co4+ and Fe3+ → Fe4+ (as well as potentially to light scattering from nucleated crystallites, grain boundaries, and a roughened surface). The STC35 crystallization temperature is consistent with work by Souza et al., who reported crystallization of SrxCo1-xTiO3 (0≤x≤0.3) at around 450-600 ºC 62. After this in situ crystallization process, the enhanced kchem in the present study may be attributed to a number of potentially synergistic structural changes across multiple length scales. Contraction during low temperature crystallization may open up new pristine surfaces absent of Sr segregation or impurity poisoning, increase the surface area for oxygen exchange, and/or expose triple phase boundaries (YSZ substrate – film – gas phase interface), which might have higher activity for oxygen exchange. The TEM micrographs of Figure 3 show some of these features in the in situ crystallized films, i.e., apparent higher surface area and presence of triple phase boundaries, consistent with increased roughness observed by scanning probe microscopy in Figure 1. Angleresolved XPS results also show that in situ crystallized films have a lower surface Sr concentration compared to films grown already crystalline; see Figure S4(a) in the supporting information for the case of perovskite STC35 and our previous work32 for the case of perovskite STF35. Additionally, the crystallization process itself might modify carrier concentrations via oxidation and possibly increase the ionic and electronic mobility in the oxide, further facilitating kinetics. These hypotheses are the subject of our ongoing, further in-depth investigations. By contrast, the PCO10 thin films did not exhibit enhanced oxygen exchange kinetics after the post-annealing process, due to the already crystalized state even in films grown at 25 ºC by PLD, as discussed above. 4.3 Extension of the OTR technique In this work, the OTR technique has been successfully applied to quantify the oxygen surface exchange coefficient of new compositions, such as perovskite STC35 and Ruddlesden-Popper phase ACS Paragon Plus Environment


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STF35, for the first time. Through monitoring the change in thin film absorption spectra upon a change in pO2, and evaluating the relaxation time by equations [1-2] (assuming surface exchangecontrolled kinetics)

32, 37, 46, 49,

the oxygen surface exchange coefficients were extracted. In very

recent experimental and computational work on perovskite STF35 we have confirmed that optical absorption changes upon changing oxygen content can be related to changes in the Fe4+ or hole concentrations and are linearly proportional to oxygen concentration changes in the conditions of these studies

63.

Similarly, in perovskite STC35 thin films, the multivalency of Co and the

corresponding ability to change oxygen content is expected to contribute to optical absorption changes in a similar way. We hypothesize that the optical absorption increase might be correlated to the change between Co3+ and Co4+ during oxidation. Malo et al. studied the transport properties of SrTi1-xCoxO3-δ (0 ≤ x ≤ 0.9) and suggested a Co3+/Co4+ mixed valency in the structure. DecorsePascanut et al. inferred the existence of a mixed valence state of Co3+ and Co4+ in SrTi1-xCoxO3-δ (x ≤0.05) prepared in oxidizing conditions by determining the oxygen stoichiometry and the average valence of cobalt ions from X-ray and neutron diffraction data

64.

We also confirmed that Co

consists of the mixed valence state of Co3+ and Co4+ in the 800 ºC-grown STC35 film by XPS, as shown in supporting information Figure S4(b).. For RP-STF35, we anticipate that the absorption changes may be related to the variation of Fe3+ and Fe4+ concentrations, similar to the case of perovskite STF35, given that the Fe environment is similar in these two structures. A deeper investigation of the defect chemistry and optical behavior of RP-STF35 could be the focus of future studies. Comparing the different compositions that we have studied by the OTR technique in this work and our previous STF35 work, we can observe which ones exhibit faster oxygen surface exchange kinetics. The value of kchem decreased in the order STC35 > STF35 > RP-STF35 > PCO10 for 25 ºC-grown thin films after annealing when measured at 400 ºC. On the other hand, kchem decreased in the order RP-STF35 > PCO10 > STF35 > STC35 for 750-800 ºC-grown thin films after annealing, again when measured at 400 ºC. These different trends highlight the importance of processing route and thermal history for surface exchange kinetics. 4.4 Ruddlesden-Popper Phase STF35 This work appears to be the first report of synthesizing the Ruddlesden-Popper Sr2Ti0.65Fe0.35O4±δ phase, to the authors’ knowledge. The XRD pattern of a bulk RP-STF35 pellet is shown in the supporting information in Figure S5(a); in addition to the primary 214 (n=1) phase, it contained a ACS Paragon Plus Environment

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secondary higher order 327 (n=2) Ruddlesden-Popper phase, i.e., Sr3(Ti,Fe)2O7±δ. The An+1BnO3n+1 RP phases have been reported to undergo layer rearrangement via ion exchange and thermodynamically favored decomposition into higher order phases, e.g., near surfaces

43, 54, 65.

As

discussed in Section 4.1, in this work, the instability could be also observed in the structure of the thin films grown at 800 °C from this bulk target. These structural changes taking place during postannealing of highly crystalline films also correlated to changes in the observed surface exchange kinetics. In Figure 2(b), the high crystalline quality of the 800 ºC-grown RP-STF35 thin film was demonstrated by XRD, albeit with apparently split peaks, which we attributed to the presence of both the (103) Sr2(Ti,Fe)1O4±δ peak and the (105) Sr3(Ti,Fe)2O7±δ shoulder, as shown in Figure S5(b). Despite the usual benefits of crystallinity for oxygen surface exchange, this particular thin film did not show optically visible oxygen exchange ability at low measuring temperatures from 300-500 ºC during the first measurement. The rapid oxygen exchange was observed optically at 550 ºC after the annealing process in the first time measurement, as shown in supplementary Figure S6(a). Then kchem continued to increase significantly after the annealing process, during the second measurement (see supplementary Figure S6(b)). This behavior is in contrast to that of highly crystalline perovskite STF35 films, which show decreasing kchem values during prolonged annealing, attributed to Sr segregation

5, 32, 66.

As discussed, the post-annealed RP-STF35 film showed a

modified XRD pattern where the peaks were not split, indicating structural evolution during the annealing. Longer-term studies of the surface exchange kinetics of the RP-STF35 films could be advantageous in the future to determine whether they avoid the intrinsic degradation and aging typical of Sr-containing MIECs. Overall, these results suggest a very promising way to prepare high performance samples at a lower temperature upon post-annealing near the crystallization temperature. More in depth studies of in situ crystallization of perovskite thin films will be presented in the near future. 5. Summary and Conclusions The impact of in situ crystallization on the oxygen surface exchange kinetics of mixed conducting thin film electrodes with different crystal structures (Perovskite STC35, Ruddlesden-Popper STF35, and Fluorite PCO10) was successfully investigated by an in situ optical transmission relaxation approach. Optical relaxations, upon changing pO2 in steps of different sizes, were observed in STC35 and RP-STF35 thin films, and were fit to determine kchem. The relative invariance of kchem to different step sizes indicated that the OTR approach with these modest pO2 steps was appropriate



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and served to demonstrate the precision and feasibility of broader application of OTR approach to these MIEC electrode candidates. In addition to determining the kchem values of these compositions for the first time at different temperatures, the OTR approach was also applied to monitor kchem during annealing-induced crystallization of amorphous films with different crystal structures. In perovskite and Ruddlesden-Popper structured films, the OTR approach enabled observation of the crystallization process during heating as well as the emergence of optically observable rapid oxygen surface exchange kinetics upon crystallization. In fact, for the perovskite STC35 and RuddlesdenPopper STF35 thin films, in situ crystallization benefitted the oxygen exchange kinetics with at least 1-2 orders of magnitude improvement, relative to crystalline films grown conventionally at high temperatures, in agreement with our prior results on perovskite STF35. On the other hand, fluorite PCO10 thin films did not show this response upon annealing, as they were already crystalline as-grown, even at 25 ºC. Acknowledgements NHP gratefully acknowledges financial support from the U.S. Department of Energy, Basic Energy Sciences through a DOE Early Career Award (grant # DE-SC00189). TC gratefully acknowledges funding from a JSPS Doctoral Fellowship (201702103). GFH acknowledges a JSPS Kakenhi Grantin-aid for Young Scientists (B) Award (No. JP16K18235) and the Progress-100 funding for Kyushu University-MIT collaboration. JM was supported for summer exchange research through the NSFPIRE X-fusion project. KS and GFH acknowledge the Center-of-Innovation program. The authors also acknowledge the International Institute for Carbon-Neutral Energy Research (WPI-I2CNER) at Kyushu University.


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Table of contents figure


Emergence of Rapid Oxygen Surface Exchange Kinetics During In

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