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Impact of Strain-Induced Changes in Defect Chemistry on Catalytic Activity of Nd2NiO4+δ Electrodes Fei Li,† Yifeng Li,‡ Huijun Chen,† Hao Li,§ Yun Zheng,‡ Yapeng Zhang,† Bo Yu,‡ Xinwei Wang,§ Jiang Liu,† Chenghao Yang,† Yan Chen,*,†,⊥ and Meilin Liu∥
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Guangzhou Key Laboratory for Surface Chemistry of Energy Materials, New Energy Institute, School of Environment and Energy, and ⊥Guangdong Engineering and Technology Research Center for Surface Chemistry of Energy Materials, School of Environment and Energy, South China University of Technology, Guangzhou 510006, PR China ‡ Institute of Nuclear and New Energy Technology (INET), Tsinghua University, 30 Shuang’qing Road, Beijing 100084, P. R. China § School of Advanced Materials, Shenzhen Graduate School, Peking University, Shenzhen 518055, China ∥ Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States S Supporting Information *
ABSTRACT: It is well known that defect chemistry plays a vital role in determining the electronic structure, ionic conductivity, and catalytic activity of metal oxides, as demonstrated in perovskite-based oxides to achieve desired functionalities. In this work, we explored the possibility of tuning the defect chemistry and hydrogen oxidation reaction (HOR) activity of Nd2NiO4+δ model thin films by controlling the lattice strain. Highly textured Nd2NiO4+δ thin films with different strain states were prepared on (110)- and (100)-oriented single-crystal yttrium-stabilized zirconium (YSZ) substrates using pulsed laser deposition. Electrochemical impedance spectroscopy results indicated that the NNO(100) film on the YSZ(110) substrate with larger tensile strain in the a−b plane and compressive strain along the c axis exhibited higher HOR activity than the NNO(110) film on the YSZ(100) substrate at 500−600 °C. The enhancement in HOR activity is attributed to the strain-induced difference in the oxygen defect concentration, as confirmed by high-resolution X-ray diffraction analysis. We believe that the correlation among the strain state, defect chemistry, and catalytic properties is helpful for rational design of more efficient electrode materials. KEYWORDS: lattice strain, hydrogen oxidation reaction, oxygen defect chemistry, perovskite-based oxides, thin film
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INTRODUCTION Perovskite-based oxides have been widely studied as the electrode materials for electrochemical energy conversion systems such as solid oxide fuel cells (SOFCs),1,2 solid oxide electrolysis cells,3,4 alkaline membrane fuel cells,5 and metal− air batteries.6,7 The electro−catalytic activity of electrodes depends sensitively on the bulk and surface composition and structure of the electrode materials.8−14 Recently, lattice strain has been used as a novel design parameter to tune the performance of electrodes.15 For instance, tensile-strained La1−xSrxCoO3−δ and Nd2NiO4+δ (NNO) (along the c axis) were reported to exhibit much faster kinetics for the oxygen reduction reaction (ORR) than the ones with compressive strain at high temperatures. Such an improvement in ORR kinetics was attributed mainly to the change in oxygen defect chemistry introduced by the external strain.16,17 In contrast, LaCoO3−δ and LaNiO3−δ with compressive strain were found to show better ORR and oxygen evolution reaction activity in liquid solution at room temperature, which were attributed to the changes in the electronic structure.18,19 As shown in the examples above, the influence of strain on the catalytic activity of the perovskite-based materials is essential; it depends not © 2018 American Chemical Society
only on the material systems but also on the reaction conditions.20−25 In this work, we explored the effect of lattice strain on hydrogen oxidation reaction (HOR) activity, a key reaction for SOFCs. Ruddlesden−Popper (RP) phase material Nd2NiO4+δ thin film was chosen as the model system because of their unique anisotropic ionic diffusion and surface exchange characteristics.26−28 It has been shown by Yamada et al. that NNO thin films grown on yttrium-stabilized zirconium [YSZ(100)] and YSZ(110) substrates exhibited very different strain states.29 The same substrates were used in this work to introduce different strain into the NNO thin film model systems. Using pulsed laser deposition (PLD), NNO(100) film with 5.37% and NNO(110) film with 4.05% tensile stain along the a−b plane were grown on YSZ(110) and YSZ(100) singlecrystalline substrates, respectively. The as-prepared films all showed good crystalline quality with flat surfaces. Electrochemical impedance spectroscopy (EIS) results showed that Received: July 15, 2018 Accepted: October 2, 2018 Published: October 2, 2018 36926
DOI: 10.1021/acsami.8b11877 ACS Appl. Mater. Interfaces 2018, 10, 36926−36932
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) AFM images of the NNO films deposited onto YSZ(110) and YSZ(100) substrates. HRXRD 2θ−ω scans of the NNO thin films deposited on (b) YSZ(110) substrate and (c) YSZ(100) substrate. The opposite red arrows shown in (b,c) represent the larger compressive strain of the NNO(100) film along the c axis than that of the NNO(110) films.
the hydrogen oxidation activity of the NNO(100) film was higher than that of the NNO(110) film. The governing reason for such difference is attributed to strain-induced variation in the oxygen defect content, which is supported by highresolution X-ray diffraction (HRXRD) results. Our results demonstrate that strain can be a tuning knob to achieve high HOR reactivity and can guide the design of high-performance catalysts.
Å, b = 5.458 Å, and c = 12.387 Å, leading to pseudocubiclattice constant with a = 3.854 Å (Supporting Information Figure S1). The a lattice constants of the NNO(100) and the NNO(110) model systems are 4.075 and 4.010 Å according to HRXRD results. With 3.854 Å as the unstrained a lattice parameter, the strain in the ab plane were quantified to be 5.73 and 4.05% for the NNO(100) and the NNO(110) films, respectively. The difference in strain state between the NNO(100) and NNO(110) films was larger than that in ref 29, which was likely due to the smaller thickness of our films (∼10 nm). HRXRD results showed that NNO model films synthesized by PLD presented different strain states along the a−b plane, with 5.37% tensile strained for the NNO(100) and 4.05% tensile strained for the NNO(110). To maintain the strain within the film, the thickness of our films was chosen to be quite small (∼10 nm). As a result, the asymmetric peaks of the films in the reciprocal space map were too weak to extract the in-plane lattice parameter. The strain states along the c axis shown in Table 1 were deduced from the measured tensile strain along the a−b plane in the NNO films, using 0.25 as the Poisson ratio value that is typical for oxides.20,30 The compressive strain along the c axis for the NNO(100) was found to be larger than that for the NNO(110), which was consistent with what was reported previously.29 To evaluate the effect of lattice strain on the HOR activity of the NNO thin films, the EIS test was conducted as a function of temperature. The experimental setup is shown in Figure 2a, with NNO thin film as the anode, YSZ single-crystal substrate as the electrolyte, and the porous Ag−YSZ as the cathode. The NNO/YSZ/porous Ag−YSZ model cell was heated up to 600 °C in 3% H2O/H2 from room temperature and held for 2 h at 600 °C to ensure that the NNO anode was fully annealed. Figure 2b shows the representative impedance spectra of the cells at 600 °C in 3% H2O/H2 and the equivalent circuit that was used to fit the Ohmic resistance (RYSZ) and polarization resistance of the films (Rp). Because the contribution of the porous cathode was much smaller than that of the thin film anode (Supporting Information Figure S2), the polarization resistance was dominated by the contribution from the HOR on the NNO anode surface. The area specific resistance (ASR) of the HOR for the NNO(100) was about 7 times smaller than that for the NNO(110) (Figure 2c), indicating that the NNO(100) film with larger tensile strained along the a−b
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RESULTS AND DISCUSSION Atomic force microscopy (AFM) results (Figure 1a) showed that the NNO films deposited on (110)- and (100)-oriented single-crystalline YSZ substrates were very smooth with roughness of less than 1 nm. Figure 1b,c shows the HRXRD 2θ−ω scan patterns, which were used to characterize the orientation and out-of-plane lattice parameters of the thin film samples. The HRXRD results indicated that the NNO films on YSZ(110) and YSZ(100) substrates were highly textured in (100) and (110) orientation, respectively (Figure 1b,c). This result is consistent with a previous report for NNO film on the single-crystal YSZ substrate grown by PLD.17,29 Table 1 compared the lattice parameters of our films with a previous report for NNO films on the same substrates (ref 29). The NNO target has an orthorhombic unit cell with a = 5.374 Table 1. Summary of the Film Orientation, a Lattice Parameter, and Strain State for the NNO(110) and NNO(100) Films in This Study and in Ref 29 NNO orientation YSZ (cubic, a = 5.13 Å) lattice parameter
bulk a/Å as-prepared a/Å strain state ab plane strain state c as-prepared a/Å in ref 29 strain state ab plane in ref 29 strain state c in ref 29 annealed a/Å plane increment a/Å
(100)
(110)
(110) 3.854 4.075 5.73% −3.82% 4.056
(100) 3.854 4.010 4.05% −2.70% 4.013
5.24%
4.13%
−3.49% 4.128 0.053 (1%)
−2.75% 4.138 0.128 (3%) 36927
DOI: 10.1021/acsami.8b11877 ACS Appl. Mater. Interfaces 2018, 10, 36926−36932
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) Schematics of the EIS test setup. (b) Representative impedance spectra of the cells with the NNO thin film anodes at 600 °C in 3% H2O/H2. The inset figure shows the equivalent circuit used for fitting the EIS data. (c,d) ASR of HOR at the NNO surface (c) and NiO−YSZ reference samples (d) as a function of inverse temperature in 3% H2O/H2. (e,f) HOR resistance for NNO anode (e) and the Ohmic resistance for YSZ electrolyte (f) as a function of operating time tested in 3% H2O/H2 at 600 °C.
plane and compressive strain along the c axis displayed much better HOR activity. Meanwhile, the Ohmic resistance (Figure S3a) of cells with the NNO(100) and NNO(110) anodes did not show noticeable difference. As the reference, the HOR activities of the NiO−YSZ films on YSZ(001) and YSZ(110) grown by PLD were also compared using the same test system (Figure 2a). As shown in Figure 2d, the NiO−YSZ on YSZ(110) and on YSZ(100) showed a similar polarization resistance for all the temperature range, indicating that there was no significant difference in HOR activity for the NiO−YSZ anode on YSZ substrates with different orientation. The Ohmic resistance was in good consistency with the ones for NNO films (Figure S3b), proving the reliability of the measurement. Having shown that the NNO(100) film with larger compressive strain along the c axis presented better HOR activity than the NNO(110) film, we further quantified the enhancement of catalytic activity for samples subjected to strain compared to strain free ones. Taking 150 nm NNO(110) film as the reference strain free sample (Figure S5 and Table S1), the 10 nm NNO(100) thin film with about −3.82% compressive strain along the c axis showed about 6−9 times better HOR activity, while the 10 nm NNO(110) thin film with −2.7% compressive strain along the c axis showed about 21−34% enhancement. More detailed quantification of the enhancement can be found in Supporting Information (Figures S4−S6, Tables S1, and S2). The stability of the NNO films as a function of operating time was tested for about 60 h in 3% H2O/H2 environment at 600 °C, as shown in Figure 2e. We can see that the ASR value for both the NNO(100) and NNO(110) films decreased during the first few hours. Such activation processes were attributed to the reduction of film in H2 gas environment and the exsolution of Ni metal particles on the surface.10 As shown in Figure 2e, the NNO(100) ASR showed a slight increase after activation while the NNO(110) remained quite stable. The NNO(100) film exhibited much larger compressive strain along the c axis than the NNO(110) film did, leading to its
better HOR activity. As a result of the larger compressive strain, the NNO(100) film may not be as stable as the NNO(110) thin film. Therefore, we believe that the increase of NNO(100) ASR is likely due to the partial relaxation of the large compressive strain in NNO(100). Despite the small increase of ASR for the NNO(100) film after the activation process, it all exhibited a lower ASR value than that for the NNO(110) over all the testing time length, indicating a better HOR activity. The Ohmic resistance for both the NNO(100) and NNO(110) films was very similar and did not show noticeable changes as a function of operating time (Figure 2f), indicating that the difference in ASR for the NNO(100) and NNO(110) films was mainly due to their different HOR activity on the surface. To understand the mechanism for the impact of strain on HOR activity, the structure, composition, and surface morphology of the NNO(100) and NNO(110) films were systematically studied using HRXRD, AFM, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The HRXRD results of the as-prepared NNO films and the films after being annealed in H2 for 2 h at 600 °C are shown in Figure 3. Both the NNO(100) and NNO(110) films maintained their crystal structure after H2 annealing. The film
Figure 3. HRXRD 2θ−ω scans of the as-prepared and annealed NNO(100) and NNO(110) thin films deposited on YSZ(110) substrate (a) and YSZ(100) substrate (b), respectively. 36928
DOI: 10.1021/acsami.8b11877 ACS Appl. Mater. Interfaces 2018, 10, 36926−36932
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a−d) SEM images of the NNO(100) film in its as-prepared states (a) and after H2 annealing (b); the NNO(110) film in its as-prepared state (c) and after H2 annealing (d); (e−h) comparison of Ni 2p and Nd 3d XPS spectra for the NNO(100) and NNO(110) films: Ni 2p spectra of the NNO films in the as-prepared states (e) and after H2 annealing (f); Nd 3d spectra of the NNO films in the as-prepared states (g) and after H2 annealing (h).
ones (Figure 4g,h), which meant that the oxidation state of Nd did not change after annealing. In contrast, the Ni 2p3/2 peak of NNO became broader and shifted to the lower binding energy after annealing, indicating a reduction of Ni valence state. The reduction of the Ni valence state was consistent with the loss of oxygen from the NNO film observed in HRXRD measurements.38−40 The reason why we saw a different chemical expansion between the NNO(100) and the NNO(110) films from HRXRD but did not observe any noticeable difference in Ni XPS spectra may be related to the different probing depth of XPS and HRXRD. While the probing depth of XPS was limited to the surface, the HRXRD probed the average changes of the whole films. The Nd/Ni ratio for the as-prepared NNO samples deduced from XPS measurements (Supporting Information Table S3) were 2.4 and 2.9 for the NNO(100) and the NNO(110) films, which were both larger than 2. This is related to A site cation segregation during film synthesis, which has been widely observed in perovskite-based materials.28,41,42 After annealing, the Nd/Ni ratio increased on both the NNO(100) and the NNO(110) surface, which may be related to the agglomerate of Ni-enriched particles on the surface, leading to the seemly increase of Nd content on the surface.28,42 The Nd-to-Ni ratio of the NNO(100) surface was slightly smaller than that on the NNO(110) surface. As demonstrated in the EIS measurement, the HOR activity of the NNO(100) surface was about 7 times better than that on the NNO(110) surface. The surface morphology and the cation valence state did not show any noticeable difference between the NNO(100) and the NNO(110) surface. Transition metal on the surface is normally considered to be the active site for the surface reaction.29,43 The Nd-to-Ni ratio of the NNO(100) surface was slightly smaller than that on the NNO(110) surface, indicating a slightly higher Ni content on the NNO(100) surface than that on the NNO(110) surface. However, such a difference in the surface composition cannot explain the nearly 7 times higher better HOR activities for the NNO(100) than the NNO(110) films. The HOR on the NNO surface can be written as43
peaks were found to shift to lower 2θ value, indicating an expansion in the a lattice parameter after reduction. This phenomenon is related to the chemical expansion that is widely observed in transition metal oxides.31,32 Because of their layered structure, RP phase oxides can accommodate both oxygen vacancies and oxygen interstitials.30,33−35 When RP phases lose oxygen, the a lattice parameter will expand due to the reduction of transition metal, while the c parameter will expand or contract depending on whether the dominate defect is oxygen vacancy or oxygen interstitial.33 In contrast to the 1% (0.053 Å) increase in the a lattice parameter for the NNO(100) film system after annealing, the a parameter for NNO(110) film system displays 3% (0.128 Å) increase. Such a difference reflect that the NNO(110) film lost more oxygen than the NNO(100) film system during annealing. The oxygen defect in the bulk NNO was reported to be dominated by oxygen interstitial.34,37 Our results indicated that the NNO (Nd2NiO4+δ) (100) film presented larger oxygen hyperstoichiometry (δ) than that for the NNO(110) film after H2 annealing. Previous works have showed that the strain along the c axis of the NNO films played a critical role in determining its oxygen defect chemistry.17,36 We found that the NNO(100) film with larger tensile strain in the a−b plane and compressive strain along the c axis lost less oxygen compared with the NNO(110) film did, which may further influence the HOR activity for NNO films. The surface morphology of the NNO films before and after annealing in H2 was characterized by SEM (Figure 4a−d) and AFM (Supporting Information Figure S7). The as-prepared NNO films showed a very smooth surface (Figures 1a and 4a,c). After annealing, both the NNO(100) and the NNO(110) films exhibited the formation of small nanoparticles, leading to the increase in the surface roughness from 0.29 to 1.64 nm for NNO(100) and from 0.24 to 1.19 nm for NNO(110) (Supporting Information Figure S7). These particles were attributed to exsolved Ni-enriched nanoparticles, which were widely observed in perovskite-based oxides after annealing in a reducing environment.10 XPS was used to probe the chemical environment and the cation valence states of the NNO film surface. As shown in Figure 4e−h, the NNO(100) and the NNO(110) did not show a noticeable difference in the Ni 2p and Nd 3d spectra, indicating a very similar surface chemical environment for Ni and Nd. The shape of Nd 3d spectrum showed no detectable change for the annealed film compared with the as-prepared
× H 2(g) + OO → H 2O(g) + 2e′ + V O••(S)
(1)
× where OO is the surface oxygen of the NNO film and V O••(S) is a doubly charged oxygen vacancy on the surface. During this reaction (1), the H2 molecule in the gas phase interacted with the oxygen on the NNO surface to form H2O molecule, which
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DOI: 10.1021/acsami.8b11877 ACS Appl. Mater. Interfaces 2018, 10, 36926−36932
Research Article
ACS Applied Materials & Interfaces left an oxygen vacancy on the surface (V O••(S)) and two electron transferring to the current collector. On the counter electrode, the oxygen got reduced on the Ag−YSZ porous cathode into oxygen ions. These oxygen ions transferred through the YSZ electrolyte to the NNO bulk and then eventually to the NNO surface. Because the surface of NNO(100) and NNO(110) films did not show large differences in the cation valence state and the chemical environment, the absorption of H2 molecular and the desorption of H2O on the NNO(100) and the NNO(110) were not likely to show large variation. It is known that the major oxygen defect in bulk NNO is oxygen interstitial.34,37 On the basis of HRXRD results, the NNO(100) lost less oxygen in H2/H2O gas environment than the NNO(110) films did, indicating a potentially more oxygen interstitial O″i presented in the NNO(100) film compared with that for the NNO(110) film. As a result, more oxygen interstitial in the NNO(100) film can be transferred to the surface to fill in the oxygen vacancies created by the reaction between H2 and surface oxygen, leading to its much higher surface activity than that of the NNO(110) film. It is needed to note that both the NNO(100) and the NNO(110) are with the ab plane, which is the fast diffusion path, exposed to the surface.26−28 We did not consider straininduced changes in the diffusion barrier of oxygen interstitial in the explanation above. Therefore, the diffusion of oxygen interstitial was considered to be similar for the NNO(100) and NNO(110) films. Furthermore, our results showed that NNO(100) lost less oxygen interstitial than NNO(110) did. Such a difference was attributed to the strain-induced changes in the oxygen defect formation energy,20 leading to different equilibrium defect concentration in the thin film at elevated temperatures. Theoretical calculation may be required to further clarify how strain changes the oxygen diffusion barrier and oxygen defect formation energy in NNO films and how such changes impact the HOR activity, which is beyond the scope of the manuscript.
could be a powerful parameter for enhancing functionalities of electrode materials for energy conversion devices.
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METHODS
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ASSOCIATED CONTENT
PLD Target Fabrication. NNO powder was synthesized by the Pechini method, with Nd(NO3)3·6H2O and Ni(NO3)2·6H2O (precursors from Macklin) as the precursor. The synthesized powder was then grinded by ball milling for 10 h and pressed to produce a 25 mm diameter target under 10 MPa. A mixing of NiO powder and YSZ powder with the ratio of 5:5 was pressed to make a NiO−YSZ target. Model Thin Film Preparation. Highly textured Nd2NiO4+δ thin films with ∼10 nm thickness (Figure S8) were deposited onto singlecrystal (100) and (110) YSZ substrates at 600 °C under an oxygen pressure of 10 mTorr with a KrF excimer laser of 248 nm wavelength. After deposition, the film was cooled to room temperature in 2 Torr oxygen pressure with a cooling rate of 5 °C/min. Electrochemical Tests. The HOR activity of the NNO(100) and the NNO(110) were evaluated using EIS measurements, with the NNO film as the anode, porous Ag−YSZ as the cathode, and singe crystal YSZ substrate as the electrolyte. A dense golden pattern was deposited on the NNO thin film by sputtering to serve as the current collector. The cells were sealed to one end of a ceramic tube with Ag paste. The cell was heated up to 600 °C in 3% H2O/H2 and held for 2 h to ensure that the NNO anode was fully annealed. The electrochemical impedance spectra were measured in the frequency range from 0.01 Hz to 1 MHz at the open circuit voltage and with an ac potential signal of 10 mV amplitude using Ivium Electrochemical Workstations. The H2 fuel flow rate was 50 mL min−1, and ambient air was used as the oxidant. Characterizations. The crystal structure of the powder and the PLD target were characterized by X-ray Diffraction (D8 ADVANCE, Bruker Corporation, Karlsruhe, Germany) with a scan rate of 5° min−1 in the 2θ range from 10° to 90°. The lattice parameter and strain state of the thin films were identified by a high-resolution fourcircle Rigaku diffractometer, equipped with 2-bounce Ge(022) channel-cut monochromator and a scintillation counter, using Cu Kα1 radiation. AFM (MultiMode 8, Bruker Corporation, Santa Barbara, California) and SEM (Nova Nano SEM 430, Holland) were used to examine the film surface morphology. The film composition and cation valence state were analyzed by XPS using monochromated Al Kα radiation (ESCALAB 250Xi, Thermo Fisher Scientific, East Grinstead, United Kingdom).
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CONCLUSIONS In conclusion, NNO thin film model systems with different strain states were synthesized by PLD. While the NNO(100) film on the YSZ(110) substrate exhibited 5.37% tensile strain along the a−b plane and −3.82% compressive strain along the c axis, the NNO(110) film on the YSZ(100) substrate presented 4.05% tensile strain along the a−b plane and −2.7% compressive strain along the c axis. Impedance analysis showed that the NNO(100) film with larger compressive strain along the c axis exhibited about 7 times higher HOR activity than the NNO(110) film. On the basis of the chemical expansion in the ab plane quantified by HRXRD, more oxygen was found in the NNO(100) film than the NNO(110) film exposed to H2/H2O environment, although the surface morphology and cation valence did not show a noticeable dependence on the strain state of the NNO films. The large difference in HOR activity between the NNO(100) and the NNO(110) films was attributed to the more oxygen interstitials in the NNO(100) film, as confirmed by the surface analysis. The excess oxygen can be transferred to the NNO surface to participate in the HOR. Our results showed that lattice strain has great impact on the oxygen defect chemistry, which in turn affects the HOR activity. Lattice stain
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b11877. XRD patterns for NNO powders and targets; X-ray reflectivity spectra for NNO(110)/YSZ(100) film; Ohmic resistance for the NNO film system and NiO− YSZ film system; polarization resistance of porous Ag− YSZ; AFM images; XPS result of Nd/Ni ratio; and quantifying strain-induced enhancement in HOR activity (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Fei Li: 0000-0002-7285-5097 Huijun Chen: 0000-0002-7320-5395 Xinwei Wang: 0000-0002-1191-8162 Jiang Liu: 0000-0002-8007-1903 Chenghao Yang: 0000-0002-3214-328X Yan Chen: 0000-0001-6193-7508 36930
DOI: 10.1021/acsami.8b11877 ACS Appl. Mater. Interfaces 2018, 10, 36926−36932
Research Article
ACS Applied Materials & Interfaces
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Meilin Liu: 0000-0002-6188-2372 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Guangzhou Science and Technology Program General Projects (no. 201707010146), the National Natural Science Foundation of China (no. 11605063 and no. 91745203), Guangdong Innovative and Entrepreneurial Research Team Program (no. 2014ZT05N200), the Science and Technology Planning Project of Guangdong Province, China (no. 2017B090916002), and the Recruitment Program of Global Youth Experts of China, the Fundamental Research Funds for the Central Universities (2018MS40).
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