Surface Chemistry of La0.99Sr0.01NbO4-d and Its Implication for

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Surface Chemistry of La0.99Sr0.01NbO4‑d and Its Implication for Proton Conduction Cheng Li, Stevin S. Pramana, Na Ni, John Kilner, and Stephen J. Skinner* Department of Materials, Imperial College London, Prince Consort Road, London SW7 2BP, United Kingdom

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ABSTRACT: Acceptor-doped LaNbO4 is a promising electrolyte material for protonconducting fuel cell (PCFC) applications. As charge transfer processes govern device performance, the outermost surface of acceptor-doped LaNbO4 will play an important role in determining the overall cell performance. However, the surface composition is poorly characterized, and the understanding of its impact on the proton exchange process is rudimentary. In this work, the surface chemistry of 1 atom % Sr-doped LaNbO4 (La0.99Sr0.01NbO4‑d, denoted as LSNO) proton conductor is characterized using LEIS and SIMS. The implication of a surface layer on proton transport is studied using the isotopic exchange technique. It has shown that a Sr-enriched but La-deficient surface layer of about 6−7 nm thick forms after annealing the sample under static air at 1000 °C for 10 h. The onset of segregation is found to be between 600 and 800 °C, and an equilibrium surface layer forms after 10 h annealing. A phase separation mechanism, due to the low solubility of Sr in LaNbO4, has been proposed to explain the observed segregation behavior. The surface layer was concluded to impede the water incorporation process, leading to a reduced isotopic fraction after the D216O wet exchange process, highlighting the impact of surface chemistry on the proton exchange process. KEYWORDS: LaNbO4, low-energy ion scattering, surface chemistry, segregation, proton transport, fuel cell electrolyte CO2-containing environments7 as well as appropriate mechanical properties,8 making them interesting contenders for proton-conducting fuel cell electrolytes. In contrast to the well-established bulk conducting mechanisms,6,9−11 the surface chemistry of the acceptor-doped LaNbO4 is not well understood. A few density functional theory (DFT) simulation studies reported the interaction between water or hydrogen and the surface of fergusonite/scheelite-type ionic condutors;12−14 however, the majority of the reports focused on stoichiometric compounds whose transport properties are very modest and did not consider the interaction between the acceptor dopant and the surface. To our knowledge, no reports on the surface chemistry of the LaNbO4 and other fergusonite/scheelite-type ionic conductors are available in the literature, despite the significant interest in utilizing them for electrochemical devices.6,15,16 Available experimental reports often contain ambiguous and sometimes conflicting information. For instance, the surface composition of BiVO4 fergusonite photocatalyst has been reported using X-ray photoelectron spectroscopy: while some claim Bi enrichment with the Bi/V ratio ranging from 2 to 2.94,17,18 others suggest a near stoichiometric BiVO4 surface.19 In considering the compatibility of the LaNbO4-based electrolytes with conventional solid oxide fuel cell cathodes, Giannici et al.20 used X-ray fl u o r es c e n c e t o e v a lu a t e t h e i n t e r f a c e b e t w e e n

1. INTRODUCTION High-temperature proton conductors have a wide range of applications including as fuel cell electrolytes, gas sensors, and hydrogen separation membranes.1 The application of hightemperature proton conductors in fuel cell technology is particularly appealing, as the fuel oxidation takes place at the cathode side and fuel dilution can be prevented.2 In addition, the relatively high operating temperature allows for improved reaction kinetics and the potential for combined heat and power units. Perovskite-based oxides such as acceptor-doped SrCeO3, BaCeO3, and BaZrO3, were among the first oxide systems investigated extensively for proton conduction.1 However, cerate-based electrolytes are not stable in wet or CO2-containing environments due to decomposition of the material into the hydroxide and carbonate, rendering them unsuitable for device applications. BaZrO3-based electrolytes, on the other hand, have much improved chemical stability, although their conductivity is generally lower than that of the BeCeO3 phase.3 In addition, poor sinterability and high grain boundary resistivity of BaZrO3 still pose further challenges for device application.4 To overcome the shortcomings of perovskite-based protonic conductors, research interest has shifted to the investigation of proton conduction in acceptor-doped fergusonite-type oxides with general formula A′1−xA″xMO4‑d (A = rare earth metals and Y, A = Ca, Sr, Ba, M = Nb and Ta) after Haugsrud and Norby demonstrated that La0.99Ca0.01NbO4‑d has a proton conductivity of approximately 0.001 S cm−1 at 900 °C under air with pH2O = 2 mbar.5,6 These niobates have good chemical stability in © 2017 American Chemical Society

Received: April 6, 2017 Accepted: August 18, 2017 Published: August 18, 2017 29633

DOI: 10.1021/acsami.7b04856 ACS Appl. Mater. Interfaces 2017, 9, 29633−29642

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ACS Applied Materials & Interfaces

successive grades of diamond spray to 1/4 μm finish to expose the stoichiometric bulk. After the polishing process, the samples were annealed under lab air at 600−1000 °C for 10 h (as shown in Figure 1). The samples were cleaned sequentially using acetone, isopropanol,

La0.98Ca0.02NbO4‑δ and La0.8Sr0.2MnO3‑δ (LSM) and, within the resolution limits of the technique, did not find any evidence of secondary phase formation. However, using micro-XANES they imply that Ca diffuses out of the electrolyte into the LSM phase under high-temperature elongated anneals (72 h). This implies changes in the surface chemistry of the interface but does not provide direct measurements of the compositional changes at the interface. Low-energy ion scattering (LEIS) is a powerful technique that has the ability to probe the chemical composition of the outermost atomic layer, although it was not until recent years that research started to utilize LEIS in the study of the surface chemistry of oxide systems.21 LEIS uses a charged primary noble gas ion (He+, Ne+, or Ar+) with energy ranging from 0.5 to 8 keV to bombard the sample surface. Noble gas projectiles that recoil from the surface are captured by a detector. On the basis of a binary collision model, the kinetic energy of the recoiled ion depends on the incident energy, instrumental geometry, and mass of the noble gas as well as the target ion on the surface.21 The extreme surface sensitivity stems from the neutralization of the noble gas ion: once penetrating deeper into the sample, the noble gas has a high chance to be neutralized and thus will not be detected.21 In this work we present the surface chemical composition of 1 atom % Sr-doped LaNbO4 (LSNO) determined using LEIS. In particular, its dependence on annealing temperature and annealing time has been addressed. Isotopic exchange measurements, using D216O as a tracer, have been conducted to evaluate the impact of surface chemistry on the proton transport properties in LSNO.

Figure 1. Schematic showing the temperature profile of the current study. Samples were sintered at 1400 °C and then polished to 1/4 μm surface finish. Polished samples were annealed at 600−1000 °C under lab air after which their surface composition were analyzed using SIMS and LEIS. Samples were then exchanged in D216O, and the surface isotopic concentration was characterized. Exchange temperature is set to be 450 °C to prevent any additional segregation during the wet anneal experiment.

and deionized water prior to the LEIS experiment. Surface chemical composition analysis of the outermost atomic layer of the annealed samples was carried out using a LEIS instrument (Qtac100, ION-TOF GmbH). A 7 keV 20Ne+ primary beam was used which allows the Sr and Nb binary collision peaks to be separated. The instrument is also equipped with a 0.5 keV Ar+ sputtering gun, which was used for depth profiling. The primary gun is directed normal to the sample surface, whereas the sputter gun has a 59° incidence angle. Scattered signals at the 145° azimuth angle were gathered using a double-toroidal energy analyzer. Due to the high surface sensitivity of LEIS, the samples were cleaned again in situ using low-dose Ar+ sputtering (∼6 × 1014 ions/ cm2, Figure S2) to eliminate adsorbed surface contaminants such as F, Si, and Na after being transferred into the instrument. A vacuum of 10−8 mbar was maintained during the LEIS analysis. 2.5. D216O Isotopic Exchange. To investigate the impact of surface chemistry on the proton transport properties, isotopic exchange using D216O as a tracer was conducted. The samples were first annealed under a mix of water vapor (pH2O = 105 mbar with normal isotopic abundance) and oxygen (pO2 = 200 mbar, no balance gas) for a period that was at least 10 times the planned exchange time. This step was to establish the chemical equilibrium between the sample and the gas phase. Samples were then exchanged at 450 °C in D216O-enriched gas (pD216O = 105 mbar, isotope fraction 95%, Isotec) with pO2 = 200 mbar for 1 and 2 h, respectively. The exchange samples were quenched to room temperature, and their surface isotopic concentration was measured using a secondary ion mass spectrometer (TOF.SIMS,5 ION-TOF GmbH). A 2 keV Cs+ ion source with nominal current of approximately 110 nA was used as the sputtering gun, while a 25 keV Bi+ primary beam was used for analysis. The sputtering beam current was measured both before and after the sputtering process and showed good consistency, indicating a constant sputtering rate. The sputtering crater size was set to 300 × 300 μm2, and a smaller area (150 × 150 μm2) at the center of the crater was analyzed. The only detectable secondary ions with D tracer were OD− (mass = 18.0085 u23) and D− (mass = 2.0136 u23), and the surface isotopic fraction was calculated according to eq 2.1

2. EXPERIMENTAL SECTION 2.1. Solid State Synthesis. One atom percent Sr-doped LaNbO4 was prepared via a solid-state reaction. All starting powders were examined with XRD to ensure that they were single phase. Stoichiometric amounts of starting powders (La2O3 (99.9%), Nb2O5 (99.9%), SrCO3 (99.9%), all from Sigma-Aldrich) were weighed and then ground together. Prior to the mixing, the La2O3 and SrCO3 powders were treated at 1000 and 350 °C, respectively, to remove moisture. The powder mixtures were ball milled for 24 h in acetone with zirconia balls to achieve homogeneous mixing. The mixed powder was first calcined under lab static air at 1200 °C for 10 h and then crushed milled and calcined again at 1400 °C for 10 h. The chemical composition of the powder was examined using X-ray fluorescence (XRF), and the results are presented in Figure S1. To produce pellets, approximately 0.5 g of powder was weighed and uniaxially pressed under 360 MPa pressure, followed by isostatic press at 300 MPa for 1 min. All samples were sintered at 1400 °C in static air for 12 h with the heating/cooling rate set at 10 °C/min. 2.2. X-ray Diffraction (XRD). Room-temperature XRD data were gathered using a PANalytical X’Pert Pro MPD with Cu Kα radiation in the 2θ range from 13° to 65°. The structure of the materials was determined using the Le Bail method with the Jana2006 refinement software suite,22 and the lattice parameters of the fergusonite LaNbO4 structure (space group I2/c ICSD no. 73390) were used as the initial input for the refinement. 2.3. Transmission Electron Microscopy (TEM). To study the chemical composition of the grain boundary, lamellar samples were prepared using a dual-beam system (FIB-SEM, Helios Nanolab 600). A JEM-2100F TEM/STEM (JEOL) operating at 200 kV equipped with an energy-dispersive X-ray spectrometer (EDX, Oxford Instruments) was used for microanalysis. A probe diameter of ca. 1 nm was used for analysis. 2.4. Low-Energy Ion Scattering (LEIS). To reduce the surface roughness in preparation for the LEIS measurement, the sintered samples were first ground with SiC paper and then polished with

C(0, t ) =

IOD− IOD− + IOH−

(2.1)

where IOD− and IOH− represent the intensity of the corresponding secondary ion signal. The OD− signal was chosen due to its relatively 29634

DOI: 10.1021/acsami.7b04856 ACS Appl. Mater. Interfaces 2017, 9, 29633−29642

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ACS Applied Materials & Interfaces high intensity (Figure S3). The normalized isotopic fraction at surface C′(0,t), is defined in eq 2.2

C′(0, t ) ≡

LSNO, within instrumental detection limits, has been successfully synthesized. The refined lattice parameters (summarized in Table 1) are in good agreement with ref 36. The reported cell volumes (Table 1) show little dependence on the Sr concentration despite the ionic radius difference between the Sr2+ and the La3+ (1.26 and 1.16 Å for Sr2+ and La3+, respectively, in 8-fold coordination28). 3.2. Grain Boundary Structure and Chemistry of La0.99Sr0.01NbO4‑d. The grain boundary of an as-sintered La0.99Sr0.01NbO4‑d sample was studied using transmission electron microscopy. The high-resolution TEM (HRTEM) image of the grain boundary structure is shown in Figure 3. The incident beam is parallel to the [101] zone axis of the grain crystallized in the I2/c space group on the right side of the image (its diffraction pattern shown in the inset). The incident beam direction is in a much higher index zone axis of the grain on the left side of the image. The d spacing along the (020) plane was measured to be 5.77(10) Å (shown in the inset), which agrees very well with the d spacing obtained from the Le Bail fitting of the X-ray diffraction data, 5.76 Å. The interface between the two grains, highlighted in the inset, is well crystallized, and no glassy grain boundary phase was observed. Similarly, no secondary phase precipitation was found in the grain boundary region. Point analysis of the chemical composition was carried out using EDX in STEM mode. The EDX spectra are reported in Figure 4, whereas their corresponding analysis positions are marked on Figure 3. The intensity of the Sr Lα peak and Sr Kα (theoretical peak positions at 1.806 and 14.165 keV, respectively) decreases steadily on moving away from the grain boundary and becomes constant when approximately 20 nm away from the grain boundary core. The EDX spectra suggest an enrichment of Sr dopant toward the grain boundary. The copper signal in the spectra resulted from the copper sample holder, and the Ga signal originated from Ga implantation during the ion-milling process for TEM lamellae fabrication. The conductivity of the acceptor-doped LaNbO4 is constrained by the grain boundary with a specific grain boundary conductivity of 10 orders of magnitude lower than that of the bulk.29,30 Several authors have speculated that the reduced grain boundary conductivity for proton conductors is related to the formation of a space charge layer at the grain boundary; a positive charged grain boundary core forms at the grain boundary, resulting in a proton depletion zone in which the proton charge carriers have reduced mobility.10,30 For the acceptor-doped LaNbO4, the space charge theory further predicts that the negatively charged alkaline earth dopant ′ ) segregates toward the grain boundary core and partially (ALa compensates for the positively charged grain boundary core.31 Only a few authors have attempted to characterize the crystal structure and composition of grain boundary of the acceptordoped LaNbO4. Using high-resolution STEM and EELS, Palisaitis et al.31 reported the formation of a Sr-enriched cubic secondary phase in the Sr,Al-co-doped La0.99Sr0.01Nb0.99Al0.01O4‑d. Fjeld et al.30 investigated the chemical composition of 0.5 atom % Sr-doped LaNbO4 along the grain boundary and concluded that the grain was free from any glassy secondary phase; however, chemical analysis across the grain boundary was not reported. This is the first report on the chemical composition of the La0.99Sr0.01NbO4‑d system that confirms Sr segregation toward the grain boundary, as proposed by the space charge theory.

C(0, t ) − Cbg Cg − Cbg

(2.2) 23

where Cbg is the natural abundance of deuterium D, 0.0115%, and Cg is the isotopic fraction in the exchange gas, which is 95%. The normalized surface isotopic concentration corresponds to Crank’s solution to Fick’s second law of diffusion for a semi-infinite medium at x = 024 and is related to the isotopic diffusivity D* and surface exchange coefficient k* using a dimensionless parameter

h′ =

k*2 t /D* by eq 2.3

C′(0, t ) = 1 − exp(h′2 ) × erfc(h′)

(2.3)

Under the conditions when the overall isotopic concentration is low (C′(0,t) < 10%), a linear approximation between log (C′(0,t) and log h′ (eq 2.4) is predicted based on Crank’s solution.25 Rearranging eq 2.4 gives rise to eq 2.5, which describes the relationship between the surface isotopic concentration and the transport parameters under such conditions

log C′(0, t ) ≈ log h′ C′(0, t1) h′ k* ≈ 1 = 1 ′ C′(0, t 2) h2 k 2*

(2.4) D2*t1 D1*t 2

(2.5)

The sputtering rate for LEIS and SIMS experiments was calibrated by measuring the depth of the crater using an interferometer (NewView 200, Zygo). The sputtering rate was also estimated using the “The Stopping and Range of Ions in Matter” (SRIM) software suite26 (Figure S4). The overall thermal profile of the experiment including the isotopic exchange experiment is schematically shown in Figure 1.

3. RESULTS AND DISCUSSION 3.1. Crystal Structure of La0.99Sr0.01NbO4‑d. The XRD patterns of LaNbO4 and La0.99Sr0.01NbO4‑d (LSNO) are reported in Figure 2. Both LaNbO4 and LSNO were successfully fitted to a model adopting the I12/c1 space group using the Le Bail refinement method,27 corresponding to the fergusonite polymorph. No crystalline secondary phase was detected in the LSNO sample, indicating that the current dopant level (1 atom % which was confirmed using XRF, Figure S1) did not exceed the solubility limit and single-phase

Figure 2. Powder diffraction patterns of La0.99Sr0.01NbO4‑d and LaNbO4 samples (13° < 2θ < 65°); both compositions were refined using the I2/c space group (gray marks show the peak positions), and no secondary phase was detected within the resolution of the instrument. 29635

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ACS Applied Materials & Interfaces Table 1. Lattice Parameters of the LSNO and LaNbO4 Phases Using the Le Bail Method composition

a (Å)

b (Å)

c (Å)

β (deg)

cell vol. (Å3)

La0.99Sr0.01NbO4‑d (this work) LaNbO4 (this work) La0.995Sr0.005NbO4‑d (ref 43) La0.98 Sr0.02NbO4‑d (ref 43) LaNbO4 (ref 43)

5.5590(1) 5.5634(3) 5.5649(3) 5.5631(2) 5.5651(1)

11.5269(3) 11.5218(6) 11.5270(5) 11.5293(3) 11.5230(1)

5.2018(1) 5.2027(3) 5.2039(2) 5.2051(2) 5.2034(1)

93.980(2) 94.058(4) 94.045(4) 94.020(2) 94.071(1)

332.52(3) 332.67(5) 332.98(5) 333.03(4) 332.84(3)

Figure 3. HRTEM image of as-sintered La0.99Sr0.01NbO4‑d revealing the grain boundary structure; electron diffraction pattern shows the orientation of the right side grain; enlarged HRTEM image on the right side corner shows a well-crystallized grain boundary. Figure 5. LEIS surface spectra of LSNO samples after annealing under static air at 600−1000 °C for 10 h. Surface spectrum of the as-polished sample is also shown as a reference. Significant Sr segregation was observed after annealing the sample above 800 °C. Note the spectra have been shifted vertically for clarification and therefore do not represent the true ion yield.

Ca impurity, which was observed in both the as-polished and the annealed samples. Depth profiling of the sample (reported in Figure 6) revealed that the surface composition was very similar to that of the bulk. Comparing with the 600 °C annealed sample, the peak shape of the 800 °C annealed sample was slightly different: a shoulder at lower energy was observed. It is possible that the signal results from collision between the incident ions and Nb from the in-depth layers; however, this assumption is unlikely considering such a feature is missing from the spectra recorder from the 600 °C annealed and the as-polished samples. Alternatively, the signal might result from a slight enrichment of Sr on the surface, which was clearly observed for the 1000 °C annealed sample (Figure 5). After 1000 °C annealing, the Nb signal from the outermost surface was much weaker whereas a strong Sr signal was observed. On the basis of Figure 5, the onset temperature for the observed Sr segregation in La0.99Sr0.01NbO4‑d was estimated to be between 600 and 800 °C. Sr segregation was also observed in the as-sintered samples, which is unsurprising considering that the sintering temperature (1400 °C) is much higher than the highest anneal temperatures (1000 °C). Interestingly, the intensity of the Sr peak of the as-sintered sample was comparable with those annealed at 1000 °C, indicating that the surface chemistry is sensitive to the processing routine as well as the thermal history of the material. The surface spectra of the 1000 °C annealed sample after prolonged Ar+ sputtering are shown in Figure 6a. Clear reduction in Sr intensity was observed with increasing sputtering dose, while the intensity of Nb and La peaks increased gradually. After normalizing the peak intensity to the

Figure 4. EDX spectra showing the variations in chemical composition from grain boundary toward a grain interior; numbers on the spectrum correspond to the positions marked in Figure 3. Enrichment of Sr toward the grain boundary was observed using EDX analysis.

3.3. Surface and near Surface Chemical Analysis. The LEIS spectra obtained from the outermost surface of several La0.99Sr0.01NbO4‑d samples annealed at temperatures ranging from 600 to 1000 °C in static air are shown in Figure 5. In this temperature regime the material will adopt the scheelite polymorph, but it is worthwhile noting that the measurements of the surfaces were undertaken at ambient temperatures, where the material will be in the fergusonite structure. It is therefore likely that the surface chemistry corresponds to the scheelite polymorph. The surface spectrum of the as-polished La0.99Sr0.01NbO4‑d reference sample obtained using 7 keV Ne+ incident ions, which has been confirmed to have the same stoichiometry as the bulk using the LEIS depth profile technique (Figure S5), is also included in the figure. For the 600 °C annealed sample, the La/Nb peak intensity ratio is very similar to that of the as-polished sample; no Sr peak was observed within the detection limit of the instrument. Each of the samples analyzed using Ne+ was also surveyed using a 3 keV He+ beam that identified F, Si, and Na surface species that were easily removed using the Ar+ cleaning procedure discussed above (Figure S2). The survey spectra also indicated a potential 29636

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800 °C. To confirm this, the same sample sets were investigated using secondary ion mass spectrometry (SIMS), which promises better counting statistics and mass resolution. The normalized intensities of LaO+, NbO+, and Sr+ secondary ions (normalized to the total intensity), which are the species with the highest ion yields for the corresponding cations, are reported in Figure 7. All SIMS depth profiles show an initial increase in the normalized intensity which is highlighted in the hatched area in Figure 7a (for the 600 °C annealed sample, the top few atomic layers were sputtered unintentionally). This sharp change in ion yield resulted from the pre-equilibrium region in which the surface chemical composition, and thus ion yield, is constantly modified by the sputtering ion implantation.45 Therefore, the obtained signals were not representative of the composition in this pre-equilibrium region.45,46 The thickness of the pre-equilibrium region depends on the projected range of the sputtering ion and was estimated (with SRIM34) to be approximately 2.4 nm under the current beam condition (1 keV O2−). The estimated thickness agrees well with the experimental observation; for the as-polished sample, stable LaO+ and Sr+ signals were recorded outside the first 3 nm layers (Figure 7a), indicating a stoichiometric surface composition, which is consistent with the LEIS depth profile reported in Figure 6b. The NbO+ signal, on the other hand, decreased continuously with sputtering dose, which might relate to the sputtering process, although the actual cause of this trend is unclear. As with the as-polished sample, steady Sr+ and LaO+ signals were observed for the 600 °C annealed sample, indicating this temperature was too low to initiate cation diffusion toward the surface, again showing good consistency with the LEIS results, in Figure 5. For the 800 °C annealed sample, a clear decrease in the Sr+ intensity was observed outside of the pre-equilibrium region; increasing the annealing temperature saw a sharper drop in Sr+ intensity for the 1000 °C annealed sample, indicating a higher Sr surface concentration (note all the intensity is plotted on a logarithmic scale). The double-hatched area in Figure 7d corresponds to the bulk region predicated by LEIS; a small but non-negligible downward trend was observed for the normalized Sr+ intensity in the 1000 °C annealed sample. This trend is difficult to detect in the LEIS depth profile due to the low primary ion yield with low Sr concentration (i.e., the last two spectra in Figure 6b). It is therefore speculated that the bulk Sr concentration was underestimated using the LEIS data, and a slightly higher Sr concentration on the surface is expected. It should be emphasized that SIMS is a qualitative technique and the ion intensity cannot be directly converted to surface cation concentration due to matrix effects.47 Nevertheless, good consistency of the segregation onset temperature as well as segregation layer thickness was found between the LEIS and the SIMS data. To investigate the dynamics of the segregation process, various La0.99Sr0.01NbO4‑d samples were annealed at 1000 °C from 10 to 40 h; the resultant LEIS surface spectra and depth profiles are reported in Figures 8 and 9, respectively. The surface spectra show little variation with extended annealing time: no significant change in peak intensity was observed when extending the annealing time from 10 to 40 h. An additional peak at 3500 eV was observed for those samples annealed for 20 h and longer, which corresponds to Sn or Sb. This peak most likely originates from contamination introduced during annealing. Unlike the surface spectra shown in Figure 5, which highlight the sensitivity of segregation behavior to annealing

Figure 6. (a) LEIS depth profile of a 1000 °C annealed sample showing the evolution of chemical composition as a function of sputtering dose, and (b) normalized cation ratio revealing a Srenriched but La-deficient layer approximately 6−7 nm thick at the surface. Horizontal lines in b showcase the bulk composition, whereas vertical line signal the depth where the bulk stoichiometry has been reached.

bulk peak intensity, the cation ratio between A and B sites as a function of sputtering dose is plotted in Figure 6b. Two distinguished regions are identified (separated by the vertical dashed line in Figure 6b): a Sr-enriched but La-deficient layer of approximately 6−7 nm thick was observed in the near surface layers. Further sputtering resulted in the plateauing of the cation ratio, which soon became indistinguishable from the nominal bulk composition. Comparing with the bulk (shown as horizontal lines in Figure 6b), the Sr/Nb cation ratio is about 2 times higher at the very surface, whereas the La concentration is much lower, leaving the surface A site deficient in comparison with the La0.99Sr0.01NbO4‑d phase, giving a nominal composition close to La0.3Sr0.02NbOx. It is of course feasible that this nominal composition could also include a contribution from the Ca content observed in the LEIS spectra, although the actual concentration of the surface Ca requires further investigation. Indeed, the segregation behavior may also be modified by the presence of light elements on the surface (such as Na and Si); however, such effect is beyond the scope of this report as the current investigation focuses on the effect of annealing temperature on the surface chemistry in the LSNO system. As mentioned above, the onset temperature for Sr segregation in LSNO was estimated to be between 600 and 29637

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Figure 7. SIMS depth profiles of (a) as-polished and (b) 600, (c) 800, and (d) 1000 °C annealed La0.99Sr0.01NbO4‑d samples. Segregation of Sr toward the surface was observed in sample annealed above 800 °C, in good agreement with the LEIS data (Figure 5). Hatched region in a and c is the pre-equilibrium region within which the ion yields vary significantly with the implant of sputtering. Double hatched region in d is the bulk composition estimated based on the LEIS results in Figure 6; slight decrease in Sr concentration was observed within this region indicating that the surface Sr concentration might be underestimated in the LEIS result.

Figure 8. (a) LEIS surface spectra of La0.99Sr0.01NbO4‑d annealed at 1000 °C for 10−40 h, and (b) their corresponding surface cation ratios. Peaks at 3500 eV (marked by asterisks in a) correspond to Sn and Sb, which probably originate from furnace contamination. No significant change in the surface composition was observed with prolonged annealing time, indicating the surface reaches equilibrium after 10 h annealing at 1000 °C. Error bars correspond to the standard deviation of the measurements from multiple sites.

However, a similar trend was observed between different annealing times; after normalizing the cation intensity to the concentration of Nb, the Sr surface concentration was estimated to be approximately 2 atom %, which then gradually decreased to the bulk level with an increasing sputtering dose. No clear dependence of the surface composition on the annealing times was found in the current study (reported in Figure 8b). In addition, segregation layers of similar thickness (approximately 6−7 nm) formed with varied annealing time,

temperature, the annealing time has limited impact on the segregation behavior. This observation indicates that the Sr segregation surface layer reaches a stable state after 10 h annealing at 1000 °C. The segregation dynamics are confirmed with the depth profiles shown in Figure 9. Overall, the [La]/[Nb] cation ratio showed little variation with prolonged annealing times, albeit the 10 h annealed sample seemed to have a slightly lower [La]/ [Nb] ratio in the 1−2 nm layer. The [Sr]/[Nb] ratio, on the other hand, is more scattered, due to the low Sr intensity. 29638

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despite the Sr segregation, the surface is overall A-site deficient and Nb has a higher surface coverage than the stoichiometric bulk composition. One of the major differences between the perovskite structure and the fergusonite LSNO is the tolerance to dopants. Perovskite-related structures have the ability to accommodate high levels of substitution (for instance, 20 mol % or even higher A site alkaline earth doped were reported for those investigated in refs 33 and 34) by BO6 octahedral tilting and distortion,39 whereas the acceptor dopant solubility in LaNbO4 is relatively low.11 The solubility of Sr has been reported to be approximately 1 atom %, above which a second phase starts to precipitate.40 It is therefore proposed that in addition to the size and charge mismatch between the dopant and the lattice ion, which has been well documented to promote dopant segregation,35,36 phase separation at the surface, due to the increased Sr concentration and its low solubility in LaNbO4,40 also contribute to the segregation behavior observed in the LSNO system. Analogy can be drawn between the LSNO and the YSZ systems; in the latter case, de Ridder41 proposed the formation of a Y-deficient monoclinic zirconia phase, not the stabilized bulk tetragonal phase, at the near surface region of the sample, as a result of the Y segregation toward the outermost surface layer. The phase separation argument was also used to explain the La surface termination in La2NiO4: Wu et al.42 studied the stability of La2NiO4 from 300 to 1373 K and concluded that the decomposition of La2NiO4 into a higher order RP phase and La2O3 was thermodynamically favored. Experimental observations of higher order RP phases at the near surface region are consistent with this simulation.37 The segregation process in the LSNO system is tentatively proposed as follows: a stoichiometric surface, free from segregation, is obtained after polishing, although the grain boundary of the sample (the gray layer in between the grains as shown in Figure 10a) is still enriched with dopant, possibly due to the space charge effect.10,30 The surface analysis of the aspolished sample as well as the EDX analysis of grain boundary composition shown in Figure 4 supports this proposition. The stoichiometric surface however is not stable, promoting the diffusion of Sr toward the surface when samples are annealed at temperatures above 800 °C. Diffusion along the grain boundary might be the predominant pathway due to the relatively high Sr concentration (Figure 4) as well as the highly defective structure43 (solid arrows in Figure 10b). Upon arriving at the surface, the additional Sr drives the phase separation into a

Figure 9. LEIS depth profile of La0.99Sr0.01NbO4‑d annealed at 1000 °C for 10−40 h; segregation behavior did not vary significantly with increasing annealing time.

again confirming that the surface reaches an equilibrium state in less than 10 h when being annealed at 1000 °C. To our knowledge, this is the first report on the surface chemistry and Sr segregation in substituted LaNbO4. The onset temperature for segregation (between 600 and 800 °C) and the segregation dynamics in the LSNO material are slower than observed in perovskite-based materials. For instance, Téllez et al. reported a significant increase in A-site coverage after annealing the polished GdBaCo2O5+d at 400 °C for 1 h.32 Extending the annealing temperature to 8 and 24 h however showed little increase in A-site surface coverage.33 The slow segregation dynamics in LaNbO4 might reflect a lower diffusivity of Sr in the fergusonite phase in comparison with the scheelite phase as the onset temperature appears to be above the fergusonite/scheelite transition temperature, although no literature data is available for comparison. 3.4. Proposed Segregation Mechanism. Sr dopant segregation has been extensively reported in other oxides such as Ruddlesden−Popper (RP) phases34 and various perovskite phases. 35,36 In those compositions, the Sr segregation was often accompanied by A-site termination. Indeed, A-site-preferred termination was reported in stoichiometric phases even without a Sr dopant, as observed in polycrystalline RP-type La2NiO4,37 PrLaNiO4,38 and doubleperovskite GdBaCo2O5+d, PrBaCo2O5+d32 among others. The LSNO system on the other hand behaves quite differently:

Figure 10. Schematics demonstrating the proposed segregation mechanisms in La0.99Sr0.01NbO4‑d. (a) As-polished surface is stoichiometric and free from segregation; although the grain boundary is still enriched with Sr (b) the annealing process allows the localized dopant to diffuse toward surface, provoking the formation of Sr-enriched impurity layers on the surface. Reproduced and adapted with permission from ref 48. Copyright 2013 Cambridge University Press. 29639

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the surface exchange coefficient is predicted, according to eq 2.5, after the Sr segregation layer has developed. It should be emphasized however that the observed exchange behavior is inevitably affected by the presence of surface contaminants such as F, Si, Na, and Ca. The absence of La or Nb on the aspolished surface (Figure S2) indicates high surface coverage by the contaminant, prior to the surface sputter cleaning, which is known to limit the surface oxygen exchange activity in yttriastabilized zirconia.45 However, the significant difference in the surface isotopic fraction (reported in Figure 11), despite the surface contaminants, would confirm that the Sr-enriched surface layer indeed has an impeding effect on the proton surface exchange. Regardless of the surface chemistry composition, the normalized isotopic fraction of LSNO after wet exchange was low (less than 5%), suggesting that the proton diffusion is limited by the surface exchange process. Surface-limited diffusion was often reported for fast ionic conductors such as YSZ45 and the LAMOX family of materials. Liu et al. reported a “flat” tracer profile similar to that reported and very low (less than 1%) normalized isotopic fraction in LAMOX.46 The Sr-enriched surface layer might impede the surface exchange process, which leads to a low proton intake of the Srenriched surface layer. The reduced surface isotopic concentration might be linked with the low proton conductivity of the surface phases. For instance, the total conductivity of Sr2Nb2O7, one of the potential surface phases based on the phase diagram by Huang et al.,44 was reported to be mixed electronic and ionic with σ = 3.2 × 10−5 S cm−1 at 1000 °C, although no proton conduction was recorded.47 The conductivity data for other potential impurity phases such as Sr2Nb5O9 and Sr4Nb17O26 are not available. Proton conduction was reported in Sr4.5Nb1.5O8.25 and Sr4Nb2O9;49 however, the high Sr/Nb ratio is inconsistent with the surface composition obtained from the LEIS data (Figure 8b), making them unlikely candidates for the surface phase. The proposed blocking effect of the Sr-enriched phases might be understood in terms of reduced oxygen vacancy concentration. The formation of impurities eliminates the oxygen vacancies in the La0.99Sr0.01NbO4‑d fergusonite phase and thus blocks the water uptake pathway between the gas phase and the oxide surface, a process similar to those perovskite-based ionic conductors.34,50 This implies that Sr content in the LaNbO4 may be detrimental to proton transport and further implies that the proton transport in the bulk material requires very low defect concentrations, all of which require further investigation. Unfortunately, the transport parameters were not obtained from the diffusion measurements, due to the low isotopic fraction coupled with data scatter in the OD− signal. Exchange conditions should be optimized to gain a better understanding of the interplay between the surface chemistry and the diffusion process.

mixture of LaNbO4, Sr2Nb2O7 and other Sr- or Nb-enriched phases, according to the SrO-La2O3−Nb2O5 ternary phase diagram,44 which explains the high Nb concentration in the near surface layers. The Sr2Nb2O7 secondary phase has been reported by Mokkelbost after sintering the sample with nominal composition La0.98Sr0.02NbO4‑d at 1500 °C for 6 h.40 It should be noted that the presence of Ca on the surface would alter the surface stoichiometry and thus the segregation behavior. It is possible that the phase separation on the surface would be altered by the Ca impurity. 3.5. D216O Isotopic Exchange−Implication of Surface Segregation on Proton Transport. The impact of surface segregation on the proton transport properties was investigated using the D216O isotopic exchange technique. After the wet exchange experiment, the cation distribution was characterized using SIMS and confirmed the presence of a Sr-enriched surface layer (Figure S6). The normalized isotopic fraction at the surface as a function of annealing conditions was characterized, and the results are plotted in Figure 11. Overall, prolonged exchange times lead to

Figure 11. Normalized surface tracer concentration after exchange at 450 °C for various sample conditions and exchange times. As-polished samples which are free from Sr segregation have a higher surface isotopic concentration than their annealed counterparts. Error bars are calculated using the standard dead-time Poisson correction methodology in which the secondary ion signal is first estimated. Standard error propagation methodology is then applied to estimate the variations in normalized isotopic concentration.

higher isotopic concentration, which is expected according to eq 2.5. For the as-polished samples, the isotopic fraction after 2 h exchange was about 1.5 times higher than that exchanged for 1 h (5.0% and 3.3%, respectively), which is in good agreement with eq 2.5, which predicts a 1.4 times increment. Under the current exchange conditions, the isotopic fraction of the aspolished samples is consistently higher than the annealed counterparts. Coupled with composition analysis, it is proposed that the presence of a Sr-enriched layer is blocking the wet exchange process. Interestingly, the 800 °C annealed samples have similar surface isotopic fractions as the 1000 °C annealed sample, although higher Sr surface coverage was confirmed for the latter (Figure 5). It seems that the relatively small amount of Sr segregation is sufficient to suppress the proton surface exchange process, highlighting the importance of the composition at the outermost layer. Assuming the surface layer does not alter the tracer diffusivity D*, a 50% reduction of

4. CONCLUSION Single-phase LSNO was successfully synthesized using solid state reaction, and refined lattice parameters from the bulk material were in good agreement with the literature reports. Chemical analysis using STEM-EDX showed that the grain boundaries were enriched with Sr, providing further experimental evidence for the existence of a space charge layer in the acceptor-doped LaNbO4. The surface chemistry of LSNO and its evolution with anneal conditions were investigated using LEIS and SIMS. A Srenriched but La-deficient surface layer approximately 6−7 nm 29640

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(5) Haugsrud, R.; Norby, T. Proton Conduction in Rare-Earth Ortho-Niobates and Ortho-Tantalates. Nat. Mater. 2006, 5, 193−196. (6) Haugsrud, R.; Norby, T. High-Temperature Proton Conductivity in Acceptor-Doped LaNbO4. Solid State Ionics 2006, 177, 1129−1135. (7) Scholten, M. J.; Schoonman, J.; van Miltenburg, J. C.; Oonk, H. A. J. Synthesis of Strontium and Barium Cerate and Their Reaction with Carbon Dioxide. Solid State Ionics 1993, 61, 83−91. (8) Kreuer, K. D. On the Development of Proton Conducting Materials for Technological Applications. Solid State Ionics 1997, 97, 1−15. (9) Wachowski, S.; Mielewczyk-Gryń, A.; Zagórski, K.; Li, C.; Jasiński, P.; Skinner, S. J.; Haugsrud, R.; Gazda, M. Influence of SbSubstitution on Ionic Transport in Lanthanum Orthoniobates. J. Mater. Chem. A 2016, 4, 11696−11707. (10) Norby, T. Proton Conduction in Solids: Bulk and Interfaces. MRS Bull. 2009, 34, 923−928. (11) Vullum, F.; Nitsche, F.; Selbach, S. M.; Grande, T. Solid Solubility and Phase Transitions in the System LaNb1‑xTaxO4. J. Solid State Chem. 2008, 181, 2580−2585. (12) Kepaptsoglou, D.-M.; Hadidi, K.; Løvvik, O.-M. M.; Magraso, A.; Norby, T.; Gunnæs, A. E.; Olsen, A.; Ramasse, Q. M. Interfacial Charge Transfer and Chemical Bonding in a Ni-LaNbO4 Cermet for Proton-Conducting Solid-Oxide Fuel Cell Anodes. Chem. Mater. 2012, 24, 4152−4159. (13) Cooper, T. G.; de Leeuw, N. H.; Leeuw, N. H. De. A Combined Ab Initio and Atomistic Simulation Study of the Surface and Interfacial Structures and Energies of Hydrated Scheelite: Introducing a CaWO4 Potential Model. Surf. Sci. 2003, 531, 159−176. (14) Yan, F. Y.; Liu, Z. G.; Ouyang, J. H.; Yan, M. F. First-Principles Calculations of Water Dissociation on the Oxygen-Deficient (010) Surface of Fergusonite-Type LaNbO4 Crystal. Int. J. Hydrogen Energy 2014, 39, 1457−1462. (15) Magrasó, A.; Fontaine, M. L.; Larring, Y.; Bredesen, R.; Syvertsen, G. E.; Lein, H. L.; Grande, T.; Huse, M.; Strandbakke, R.; Haugsrud, R.; Norby, T. Development of Proton Conducting SOFCs Based on LaNbO4 Electrolyte - Status in Norway. Fuel Cells 2011, 11, 17−25. (16) Esaka, T.; Mina-ai, T.; Iwahara, H. Oxide Ion Conduction in the Solid Solution Based on the Scheelite-Type Oxide PbWO4. Solid State Ionics 1992, 57, 319−325. (17) Sinclair, T. S.; Hunter, B. M.; Winkler, J. R.; Gray, H. B.; Müller, A. M. Factors Affecting Bismuth Vanadate Photoelectrochemical Performance. Mater. Horiz. 2015, 2, 330−337. (18) Zhao, Z.; Dai, H.; Deng, J.; Liu, Y.; Au, C. T. Effect of Sulfur Doping on the Photocatalytic Performance of BiVO4 under Visible Light Illumination. Cuihua Xuebao/Chin. J. Catal. 2013, 34, 1617− 1626. (19) He, H.; Berglund, S. P.; Rettie, A. J. E.; Chemelewski, W. D.; Xiao, P.; Zhang, Y.; Mullins, C. B. Synthesis of BiVO4 Nanoflake Array Films for Photoelectrochemical Water Oxidation. J. Mater. Chem. A 2014, 2, 9371−9379. (20) Giannici, F.; Canu, G.; Gambino, M.; Longo, A.; Salomé, M.; Viviani, M.; Martorana, A. Electrode-Electrolyte Compatibility in Solid-Oxide Fuel Cells: Investigation of the LSM-LNC Interface with X-Ray Microspectroscopy. Chem. Mater. 2015, 27, 2763−2766. (21) Brongersma, H. H.; Draxler, M.; de Ridder, M.; Bauer, P. Surface Composition Analysis by Low-Energy Ion Scattering. Surf. Sci. Rep. 2007, 62, 63−109. (22) Petříček, V.; Dušek, M.; Palatinus, L. Crystallographic Computing System JANA2006: General Features. Z. Kristallogr. Cryst. Mater. 2014, 229, 345−352. (23) Audi, G.; Wapstra, A. H.; Thibault, C. The Ame2003 Atomic Mass Evaluation - (II). Tables, Graphs and References. Nucl. Phys. A 2003, 729, 337−676. (24) Crank, J. The Mathematics of Diffusion; Oxford University Press, 1979. (25) Chater, R. J.; Carter, S.; Kilner, J. A.; Steele, B. C. H. Development of a Novel SIMS Technique for Oxygen Self-Diffusion

thick was revealed on the LSNO surface after annealing the sample at 1000 °C for 10 h. The onset of Sr segregation was estimated to be between 600 and 800 °C, and a thermodynamically stable surface, with a nominal composition close to La0.3Sr0.02NbOx, was formed after annealing at 1000 °C for less than 10 h. Unlike perovskite systems in which the Sr segregation is accompanied by A-site dominance, the surface of the annealed LSNO is overall La deficient in comparison to the bulk phase. This unique segregation behavior was explained in terms of phase separation: the as-polished stoichiometric surface is not stable and encourages Sr diffusion toward the surface during annealing. Upon arriving at the surface, the solubility limit of Sr is likely to be surpassed provoking a phase separation into a mix of LaNbO4 and other Sr- and/or Nb-enriched phases leading to the observed surface compositions. Surface isotopic concentration at the surface was characterized after exchange in a D216O-enriched environment at a temperature of 450 °C for between 1 and 2 h. A systematic decrease in isotopic concentration was observed after the Srenriched layer had developed at the surface. It is speculated that the formation of stoichiometric impurities might have eliminated the surface oxygen vacancies and thus blocks the water intake pathway between the gas phase and the oxide surface; future work is planned to further investigate the effect of surface segregation on the diffusion process.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04856. Calibration curves for XRF analysis, details of the deuterium exchange and associated SIMS data, analysis of SIMS crater depths as both a figure and tabulated data, details of the LEIS data analysis procedure, SIMS depth profiles at 450 °C, conductivity data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], tel.: +44 (0)20 7594 6782. ORCID

Stevin S. Pramana: 0000-0001-5837-7554 Stephen J. Skinner: 0000-0001-5446-2647 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.N. would like to acknowledge funding from an Imperial College Research Fellowship.

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REFERENCES

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