Strain Effect on Oxygen Evolution Reaction Activity of Epitaxial

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Strain Effect on Oxygen Evolution Reaction Activity of Epitaxial NdNiO3 Thin Films Le Wang, Kelsey A. Stoerzinger, Lei Chang, Xinmao Yin, Yangyang Li, Chi Sin Tang, Endong Jia, Mark E. Bowden, Zhenzhong Yang, Amr Abdelsamie, Lu You, Rui Guo, Jingsheng Chen, Andrivo Rusydi, Junling Wang, Scott A. Chambers, and Yingge Du ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21301 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 5, 2019

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ACS Applied Materials & Interfaces

Strain Effect on Oxygen Evolution Reaction Activity of Epitaxial NdNiO3 Thin Films Le Wang†,‡, Kelsey A. Stoerzinger*,†,§, Lei Chang‡, Xinmao Yin∥, Yangyang Li#, Chi Sin Tang∆, Endong Jia†,⊥,⁋, Mark E. Bowden¶, Zhenzhong Yang†, Amr Abdelsamie‡, Lu You‡, Rui Guo#, Jingsheng Chen#, Andrivo Rusydi∥, Junling Wang*,‡, Scott A. Chambers†, Yingge Du*,†

†Physical

and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, United States ‡School

of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore §School

of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, Oregon 97331, United States ∥Department

of Physics, Faculty of Science, National University of Singapore, Singapore 117542,

Singapore #Department

of Material Science & Engineering, National University of Singapore, Singapore 117575,

Singapore ∆NUS

Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore 117456, Singapore ⊥The

Key Laboratory of Solar Thermal Energy and Photovoltaic System, Institute of Electrical Engineering, Chinese Academy of Science, Beijing 100190, China ⁋University

of Chinese Academy of Sciences, Beijing 100190, China

¶Environmental

Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354, United States

KEYWORDS: nickelates, strain, orbital polarization, oxygen vacancy, oxygen evolution reaction, NdNiO3 ABSTRACT: Epitaxial strain can cause both lattice distortion and oxygen non-stoichiometry, effects that are strongly coupled at the heterojunctions of complex nickelate oxides. In this paper, we decouple these structural and chemical effects on the oxygen evolution reaction (OER) by using a set of coherently-strained epitaxial NdNiO3 (NNO) films. We show that within the regime where oxygen vacancies (VOs) are negligible, compressive strain is favorable for the 1 ACS Paragon Plus Environment

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OER while tensile strain is unfavorable, as the former induces orbital splitting, resulting in a higher occupancy in the 𝑑3𝑧2 ―𝑟2 orbital and weaker Ni-O chemisorption. However, when the tensile strain is large enough to promote the formation of VOs, an increase in the OER is also observed. The partial reduction of Ni3+ to Ni2+ due to VOs makes the eg occupancy slightly larger than unit, which is believed to account for this increased OER activity. Our work highlights that epitaxial-strain-induced lattice distortion and VO generation can be individually or collectively exploited to tune OER activities, which is of importance for the predictive synthesis of highperformance electrocatalysts.

I. INTRODUCTION Perovskite oxides have shown great promise in applications ranging from electronics,1-2 multiferroics,3 to electrocatalysis.4-6 Their common formula of ABO3 can accommodate a wide range of redox-active, earth abundant transition metals (B-site) and structural rare-earth or alkaline-earth metals (A-site), resulting in diverse structural, physical, and chemical properties.610

The main functional group in the ABO3 structure is the BO6 octahedra. Interfacial stain can

cause changes in the size, shape, and connectivity of the octahedral building blocks.11-12 Such structural distortions influence the orbital overlap between transition metal 3d and oxygen 2p,1314

which dictates electronic properties including the metal-to-insulator transition (MIT),15-18 and

has been linked to the reactivity of oxygen on the surface of perovskites.19-21 In addition, stain can lead to the formation of structural defects, such as isolated oxygen vacancies (VOs) or ordered oxygen vacancy channels (OVCs),22-25 which also greatly impact the physical and chemical properties of the resulting materials.24, 26 In many cases, the structural distortion and

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defect generation are strongly coupled at heteroepitaxial interfaces, and their impacts on the materials chemistry are convoluted. Recently, large efforts have been made to understand the relationship between perovskite oxide electronic structure and their activity for the oxygen evolution reaction (OER),19, 27-31 a key reaction in energy storage schemes including water electrolysis32-33 and rechargeable metal-air batteries.34 Previous reports reveal that OER activities of the perovskite oxide catalysts are highly dependent on their composition,21, 31, 35-36 transition metal ion’s electron configuration,19 surface oxygen binding energy,37 and the oxygen stoichiometry.26, 30, 38 Bockris and Otagawa39 reported an extensive study based on the perovskite oxides system, where they discovered a trend (Ni > Co > Fe > Mn > Cr) of OER activity for samples containing different transition metals. Moreover, Suntivich et al.19 predicted that the peak OER activity should be at an eg occupancy close to unity, with high covalency between transition metal 3d and oxygen 2p. Based on these previous reports,19, 39 perovskite rare earth nickelates (RNiO3, where R represents rare earth lanthanide elements) should be of great interest for OER, due to the Ni3+ valence state (3d7: 𝑡62𝑔𝑒1𝑔) and high covalency between Ni 3d and O 2p bands.40 In fact, LaNiO3 in bulk form has already been demonstrated to possess good OER activity.39 High quality nickelate films can offer the opportunity for fundamental studies to understand the relationship between electronic structure and the OER. With respect to bulk materials, thin films are highly sensitive to substrates when grown in an epitaxial manner.2, 41 Petrie et al.20 reported that compressive strained LaNiO3 thin films can drive the OER at lower overpotentials, and surpass the performance of noble metals such as Pt. With compressive strain, the eg-band center position trends toward lower energies relative to the Fermi level, which results in weaker Ni-O chemisorption and a subsequent enhancement in the OER activity.20 3 ACS Paragon Plus Environment


In our recent study,31 we have shown that NdNiO3 (NNO) films grown on tensile strained SrTiO3 (STO) substrates offer even higher OER performance than LaNiO3, as a result of the incorporation of VOs. The corresponding Ni3+→Ni2+ valence change resulting from oxygen vacancies is favorable for OER because it increases the average eg occupancy to more than one.31 For NNO, the MIT temperature (TMI) gradually shifts to lower temperatures as the strain changes from the tensile regime to the compressive regime, and eventually a single metallic phase is stabilized under higher compressive strain.42-43 The lowering of TMI with compressive strain has been attributed to covalency bandwidth broadening due to the lattice distortion,42 which should favor OER activity.19 Considering that lattice-strain-induced structural distortion and defect generation (such as VOs) are strongly coupled at the heterojunctions of NNO and a crystalline substrate (Figure 1a), it is of great interest to decouple such entanglement, and elucidate their respective effect. Here, we prepared a set of coherently strained, ultrathin (~5 nm) epitaxial NNO films by pulsed laser deposition (PLD) with controlled strain states and defect concentrations. We elucidated the effects of strain on the structural and electronic properties of NNO films, and revealed the correlation between these physical properties and OER catalysis.

II. RESULTS AND DISCUSSION ~5 nm epitaxial NNO thin films were grown by PLD on single crystalline (001)t-oriented SrLaAlO4 (SLAO), (001)pc-oriented LaAlO3 (LAO), (110)o-oriented NdGaO3 (NGO), and (001)c-oriented STO substrates (the subscripts ‘t’, ‘pc’, ‘o’ and ‘c’ represent tetragonal, pseudocubic, orthorhombic, and cubic structures, respectively). In bulk, NNO can be seen as a pseudocubic structure with apc=3.81 Å.44 Hence, NNO experiences compressive strains of 1.39% and -0.47% on SLAO and LAO, and tensile strains of +1.26% and +2.49% on NGO and 4 ACS Paragon Plus Environment

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STO (Table 1), respectively. The surface topography of NNO films was characterized by atomic force microscopy (AFM) (Figure S1). The surface roughness is ~ 0.2 nm for all the NNO films. Figure 1b shows X-ray diffraction (XRD) θ−2θ scans around (002) peak of the 5 nm thick NNO films, with extended XRD patterns presented in Figure S1. The arrows indicate the (002)pc peak of NNO, which shifts to higher 2θ angle from SLAO to STO, indicating a decrease of the out-ofplane lattice parameter (OOP). The inset of Figure 1c shows a typical reciprocal space map (RSM) of 5 nm NNO grown on SLAO, confirming that the NNO films are coherently strained. The theoretical OOPs of strained NNO films can also be estimated from the Young’s modulus, which we marked as black open squares in Figure 1c. The solid circles in Figure 1c denote the experimental OOPs of NNO films, deduced from the film peaks in the θ−2θ scans shown in Figure 1b. Compared to the pseudo-cubic lattice constant (3.81 Å, noted by red dashed line in Figure 1c) of bulk NNO, compressive strain induces the increase of OOP while tensile strain decreases OOP. The OOP of NNO on LAO matches well with the calculated fully strained OOP, indicating that NNO films grown on the low-mismatch LAO substrate is VO-free. However, the OOPs of the NNO films grown on other three substrates (SLAO, NGO, and STO) are larger than the calculated fully strained values, most likely due to the formation of VOs in these NNO films resulting from the larger lattice mismatch.31, 45-47 The large difference between experimental and calculated data found in NNO on STO (=+2.49%) indicates that the VO concentration is significantly higher than others, consistent with previous findings that the VO formation energy is lower under a tensile strain.24-25

To better understand the changes in the electronic properties of the NNO films as a function of strain, we performed X-ray absorption spectroscopy (XAS) measurements of both the Ni L-edge



and O K-edge. Strain effect on orbital occupancy can be directly probed by collecting two absorption spectra with linearly polarized light.18 Figure 2a illustrates the configuration of the experimental setup. The X-ray incident angle 90 and 30 corresponds to in-plane (Eab) and outof-plane (Ec) polarization component, respectively. The X-ray linear dichroism XLD (XLD=EabEc) provides the information about the preferred orbital occupation.13, 18, 48 Due to the strong overlap between La M4 and Ni L3 edges for NNO/SLAO and NNO/LAO (Figure S2), we focus on the Ni L2 edge only. Although the absorption intensity of Ni L2 edge is much lower than that of Ni L3 edge, previous literature has also established use of the Ni L2 edge to determine eg orbital splitting in nickelates.14,17, 49-50 Figure 2b shows the normalized XAS and the associated XLD results of the strained NNO films. The integrated XLD intensity is positive for the compressive strained NNO films, indicating preferential occupation of the 𝑑3𝑧2 ―𝑟2 orbital. The negative integrated XLD intensity for the tensile strained NNO films suggests that the preferred occupation of the 𝑑𝑥2 ―𝑦2 orbital (Figure 2c). Similar conclusions are reached with XLD analysis for both L3 and L2 edges (Figure S2). The O K pre-edge XAS spectra for NNO films were shown in Figure 2d and Figure S3. The O K pre-edge near 528 eV is a measure of the Ni 3d-O 2p hybridization that governs conduction in this system. Moreover, O K pre-edge XAS can be also used to monitor the VOs concentration.25, 30-31, 46, 51 As shown in Figure 2d, the intensities of the pre-edge decrease with  changing from -0.47% to 1.26%, and 2.49%, suggesting that the VO concentration in NNO films systematically increases as tensile strain increases, consistent with previous reports.24-25 Moreover, compared with NNO on LAO (=-0.47%), the O K pre-edge intensity of NNO on SLAO (=-1.39%) also decreases in a magnitude similar to that on NGO (+1.26%), indicating that VOs also form under larger compressive strain.


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Figure 3 shows the resistivity versus temperature (~T) curves for 5 nm NNO films grown on these four different substrates. For larger compressive strain (=-1.39%), NNO on SLAO displays metallic and paramagnetic down to the lowest temperature, consistent with the recent findings in compressive strained PrNiO3 (PNO),52 NNO,42-43, 53 EuNiO3 (ENO)16 films, where it was proposed that non-Fermi liquid (NFL) behavior occurs with the suppression of the temperature-driven MIT due to the compressive strain.42-43 For the smallest lattice mismatch (=0.47%) studied in this work, the metallic state of NNO on LAO has the lowest resistivity (~4.610-5 .cm at room temperature) and a sharp MIT with a significant hysteresis, in agreement with previous reported results.54-56 As  goes to the tensile strain side, the room temperature resistivity increases to ~6.810-5 and ~2.510-4 for NNO films grown on NGO and STO, respectively. This increase in resistivity and the broader MIT could be related to the presence of Ni2+ due to the VOs (confirmed by XRD shown in Figure 1c and XAS shown in Figure 2d).55 We next investigate the strain effect on the OER activities. NNO films grown on different substrates were electrically contacted on the front and OER activity was measured in O2saturated 0.1 M KOH. The experimental setup has been described elsewhere.31 Figure 4a shows the polarization curves for OER on these strained NNO films, indicating that compressive strain can obviously enhance OER activity, consistent with the OER trend in LaNiO3 films.20 Previous study on strained LaCoO3 films have shown enhanced OER activity trended directly with a decreased charge transfer resistance.41 The OER activity is maximum for NNO on SLAO (Figure 4a, Figure 4b and Figure S4), with activity decreasing as  changes from -1.39% to -0.47% and +1.26%. However, NNO on STO shows a higher OER activity than NNO on NGO, albeit the tensile strain is larger (+2.49%). The OER Tafel slopes (Figure 4b) are ~70 mV decade-1 for 7 ACS Paragon Plus Environment


NNO on SLAO, while ~ 130 mV decade-1 for all other strained NNO films. The shallower Tafel slope means a larger increase in current density for a given change in overpotential, revealing good kinetics and transfer of charge for NNO on SLAO. To elucidate how strain affects the OER activity of NNO films, we summarize the strain effect on the physical properties and OER activity in Figure 5. Figure 5a and Figure 5b show the room temperature resistivity and the O K pre-edge intensity of NNO films as a function of strain, respectively. The O K pre-edge intensity change is consistent with the room temperature resistivity change of NNO films. The largest resistivity reveals the highest VOs concentration in NNO on STO (+2.49%). Figure 5c shows the integrated XLD results as a function of strain. The integrated XLD intensity increases with increasing the epitaxial strain, indicating that the orbital polarization increases. Strain dependent orbital polarization observed in our NNO films is consistent with the previous reports of nickelates,18, 20 manganites,13 and strontium cobaltite thin films.57 Moreover, Petrie et al.20 calculated the asymmetry of surface orbital occupancy for LaNiO3 films, and they found that at the surface the orbital polarization is shifted toward the 𝑑3𝑧2 ―𝑟2 orbital over the entire strain range. The eg band center at the surface trended toward lower energies with compressive strain, resulting in weaker Ni-O chemisorption.20 In addition, the eg orbital splitting due to strain is strongly dependent on the NiO6 coordination structure (see the inset of Figure 5d). Previous studies reported that the change of surface BO6 octahedra can strongly impact the OER activity.37, 58 Compressive strain can shrink the Ni–O bond length and straighten the Ni-O-Ni bond angle,40 which can increase the orbital overlap and stabilize the metallic state (Figure 3). The enhancement of orbital overlap or p-d hybridization is favorable to OER.21 Considering these two effects as we mentioned above, we can understand why compressive strained NNO films show more active OER (Figure 5d). For the larger tensile 8 ACS Paragon Plus Environment

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strained case, VOs are notable (Figure 1c and Figure 2d). Compared with NNO on NGO (=+1.26%), NNO on STO (=+2.49%) shows higher OER activity (Figure 5d), which can be ascribed to the increase of VOs. As the introduction of VOs in NNO will reduce Ni3+ to Ni2+ and increase the average eg occupancy to more than unit, OER activity is enhanced.19, 31 Compared with LaNiO3 films grown on LAO (=-1.2%),20 the NNO film grown on SLAO (=-1.39%) shows higher OER activity, which could have a contribution from the small amount VOs in NNO on SLAO, in addition to changes in Ni-O overlap with strain. Further theoretical calculation work can help understand more details of OER with strain, and it can also give us more clear images which steps (mainly OH*, O*, and OOH* in the OER) are affected by strain.59

III. CONCLUSION In summary, we have investigated the strain effect on the structural and electronic properties, and directly linked them to the OER activity of NNO thin films. XRD, XAS and in-plane transport measurements reveal that in general, increasing tensile strain induces a higher oxygen vacancy content in NNO films. XLD measurements demonstrate that increasing the strain can increase the orbital polarization. Moreover, with compressive strain the eg center at the surface trends toward lower energies, resulting in weaker Ni-O chemisorption, which enhances OER activities, with NNO on SLAO (-1.39%) being the most active. However, in the regime where oxygen vacancies are non-negligible (e.g., NNO grown on STO), an increase in OER activity is again observed, as oxygen vacancies can reduce Ni3+ to Ni2+ and increase the average eg occupancy to more than one. Our study on the OER activities of NNO films with different strain states offers new understanding in designing active electrochemical catalysts based on complex oxides, in which structural distortion and defect formation are strongly coupled. 9 ACS Paragon Plus Environment


METHODS The experimental setups for NNO films growth, in-plane transport measurements, and OER measurements have been described elsewhere.31 The high-resolution XRD instrument from the beamline at the Singapore Synchrotron Light Source (SSLS) was used to check the crystallographic structures of these NNO films. The RSM measurements of these NNO films were performed by using the high-resolution XRD in EMSL. The Ni L-edges and O K-edges XAS spectra were also measured at the beamline at SSLS.

ASSOCIATED CONTENT Supporting Information This supporting information is available free of charge via the internet at http://pubs.acs.org. AFM and extended XRD -2 patterns; Ni L edge XAS; O K edge XAS; five cycles of cyclic voltammetry (CV) of 5 nm NNO films.

AUTHOR INFORMATION Corresponding Authors *Email:

[email protected]; [email protected]; [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is support by the U.S. Department of Energy (DOE), Office of Science, Early Career Research Program under Award No. 68278. The XRD reciprocal space maps measurements are support by U.S. DOE, Office of Science, Office of Basic Energy Sciences, the Division of Materials Sciences and Engineering under Award #10122. J. Wang acknowledges financial 10 ACS Paragon Plus Environment

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support from Ministry of Education, Singapore under the Grant No. MOE2014-T2-1-099. Electrochemical measurements were supported for K.A.S. by the Linus Pauling Distinguished Post-doctoral Fellowship at Pacific Northwest National Laboratory (PNNL LDRD 69319). We thank Dr. Xu He from University of Liège in Belgium for useful discussions. A portion of the research was performed using EMSL (grid.436923.9), a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research. PNNL is a multi-program national laboratory operated for DOE by Battelle.

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FIGURES AND FIGURE CAPTIONS

Figure 1. (a) Schematic models of two tensile-strained interfaces illustrating structural distortion and defect formation, respectively. Typically, the in-plane lattice parameters of a coherently strained film are physically constrained by the substrate, which leads to an inverse change in the film’s out-of-plane lattice parameter (c'), governed by the material's Poisson ratio. Moreover, under elastic tensile strain, stretching of the B-O bonds leads to the smaller overlap of the metal 3d and O 2p bands, thus decreasing the VO formation energy. The formed VOs induce the increase of the out-of-plane lattice parameter (c''). (b) XRD θ-2θ scans around the (002) peak of NNO films on different substrates. Substrates peaks and NNO films 16 ACS Paragon Plus Environment

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peaks are noted by stars and arrows, respectively. (c) OOPs of NNO films on different substrates. The dashed red line represents the pseudocubic lattice constants for bulk NNO. Black open squares represent the strained OOPs of NNO films estimated from the Young’s modulus by taking into account the

Possion’s ratio v0.23. The inset shows RSM of a 5 nm NNO film grown on SLAO.

Figure 2. (a) Schematic of experimental configuration for XAS measurements with different Xray incident angles. (b) Normalized XAS Ni L2 spectra and XLD of NNO films grown on different substrates. (c) Schematic showing the lifting of eg orbital degeneracy in strained NNO. Bulk NNO (no strain) does not show any orbital ordering. (d) O K edge spectra for NNO films grown on different substrates.



Figure 3. Temperature dependence of electrical resistivity () for 5 nm NNO films grown on these four different substrates. TMI (marked by coloured arrows) are defined as the temperatures of the upturn in the resistivity versus temperature plots.


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Figure 4. (a) Cyclic voltammetry (CV) curves for the OER at a scan rate of 10 mV s-1 in O2saturated 0.1 M KOH for the NNO films grown on different substrates. (b) Tafel plots of these four samples indicate the steady state current obtained from chronoamperometry (CA) as points with lines to guide the eye.



Figure 5. Room temperature resistivity (a) and O K pre-edge intensity (b), integrated XLD (c), and OER current measured at 1.63 V (d) of NNO films as a function of strain. NiO6 octahedra rotation under strain effect is schematically shown in the inset of (d).


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TABLES AND TABLE CAPTIONS Table 1. In-plane pseudo-cubic lattice constants, asub, of the used substrates and their corresponding lattice mismatch () with bulk NNO with pseudo-cubic lattice constant of 3.81 Å, where =(asub-3.81)/3.81. SrLaAlO4 (SLAO)

LaAlO3 (LAO)

NdGaO3 (NGO)

SrTiO3 (STO)

asub (Å)

3.757

3.792

3.858

3.905

 (%)

-1.39

-0.47

+1.26

+2.49



TOC graphic


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Strain Effect on Oxygen Evolution Reaction Activity of Epitaxial

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