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Structural Modification of Self-Organized Nanoporous Niobium Oxide via Hydrogen Treatment Kyungbae Kim,† Moon-Soo Kim,† Pil-Ryung Cha,† Soon Hyung Kang,*,‡ and Jae-Hun Kim*,† †

School of Advanced Materials Engineering, Kookmin University, Seoul 02707, Republic of Korea Department of Chemistry Education and Optoelectronics Convergence Research Center, Chonnam National University, Gwangju 61186, Republic of Korea

‡

S Supporting Information *

ABSTRACT: Niobium pentoxide (Nb2O5) is an interesting material with applications in Li battery and hybrid capacitor electrodes. The main limitation of this material is its low electronic conductivity. In this study, H2 treatment is introduced to address this issue. Self-ordered Nb2O5 films were prepared by anodizing Nb foils and subsequently treating them in a H2 atmosphere. Electron microscopy revealed that the Nb2O5 film had a hierarchical porous microstructure consisting of macropores and mesopores. X-ray diffraction analysis showed that the crystal structure could be changed by the H2 treatment compared to the air treatment. Oxygen deficiencies in the Nb2O5 film were induced by the treatment, as confirmed by X-ray photoelectron spectroscopy. Mott− Schottky analysis was performed and indicated that the electronic conductivity of the material was significantly improved by the oxygen deficiencies. Thus, the electrochemical Li storage kinetics in porous Nb2O5 films can be greatly enhanced by H2 treatment.

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INTRODUCTION Niobium pentoxide (Nb2O5) is an important functional metal oxide material because of the unique physical and chemical properties that result from its energy band structure. This material has mainly been used in the limited applications, such as gas sensors, electrochromics, and catalysts. In recent years, however, its applications have expanded to energy devices such as solar cells, rechargeable batteries, and supercapacitors, because of its various polymorphic forms and morphologies and its relative abundance in nature.1−7 Regarding battery applications, the storage of Li ions in Nb2O5 materials via electrochemical reaction in a nonaqueous solution was reported approximately 30 years ago.8,9 Since then, Nb2O5 with various crystal structures has been intensively studied as a cathode material candidate for 2 V Li rechargeable battery applications.10−13 Recently, Nb2O5 materials with a specific crystal structure were reported to have the advantage of fast Li intercalation and deintercalation reactions.14−17 These results have attracted renewed interests in applying Nb2O5 as electrode materials for hybrid capacitors such as Li-ion capacitors,18−21 which require higher power than conventional Li-ion batteries. Nb oxide materials have been synthesized as powders and thin films through a number of methods, including sol−gel, hydrothermal, electrodeposition, vapor deposition, thermal oxidation, sputtering, and anodization methods.1 Among them, the electrochemical anodization method is one of the most widespread fabrication processes for the preparation of highly porous and ordered oxide films on substrates. Self© XXXX American Chemical Society

organized oxide nanostructures can be obtained through the anodization of a range of metals under various experimental conditions.22−26 For example, Al and Ti metals can be easily transformed into nanoporous Al oxide and Ti oxide nanotube films, respectively. Recently, the electrochemical anodization of Nb metal has been investigated by many research groups.27−34 The resulting nanoporous or nanochannel Nb oxide films have shown potential applications as electrode materials for electrochemical energy storage devices, such as Li batteries and hybrid supercapacitors.35,36 Here, we used anodization to prepare three-dimensional (3D) nanoporous and self-ordered Nb2O5 films as electrode materials. The use of self-ordered porous Nb2O5 films for Liion capacitors is expected to be beneficial in terms of reaction kinetics because the porous structure can provide pores for fast ionic transport through electrolyte and relatively high surface area for charge transfer. However, our previous work revealed that the electronic transport through nanoporous Nb2O5 films can be a main limiting factor for reactions with high reaction rates.36 Stoichiometric Nb2O5 is known to be an insulator with an electrical conductivity (σ) of ∼3 × 10−6 S cm−1 and to become an n-type semiconductor (σ = 3 × 103 S cm−1) under oxygen-deficient conditions.2 In this study, we report a detailed investigation of improved Li storage kinetics in H2-treated Received: December 15, 2015 Revised: February 11, 2016

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DOI: 10.1021/acs.chemmater.5b04845 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Figure 1. Surface and cross-sectional FE-SEM images of self-organized Nb2O5 films subjected to different treatment atmospheres: (a−c) air, (d−f) Ar, and (g−i) H2.

Nb2O5 films. Nanoporous and oxygen-deficient Nb oxide films were prepared by anodizing Nb metals and then subjecting them to H2 treatment. The material and electrical characteristics of the H2-treated Nb2O5 films were thoroughly examined by electron microscopy, X-ray photoelectron spectroscopy with depth profiling, and Mott−Schottky analysis. The electrochemical properties relating to Li storage were also studied by cyclic voltammetry. Air- and Ar-treated Nb2O5 films were also prepared and used as controls.

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analyze the chemical state of the oxide. XPS in-depth profiles were obtained as the Ar-ion etching time increased to 300 s with a constant interval (60 s). Additionally, to assess the flat-band potential (VFB) and donor concentration of the surface-treated Nb2O5 films, Mott− Schottky plots (AUTOLAB/PGSTAT, 128N) with a frequency of 1 kHz were measured using a potentiostat (BioLogic VSP) equipped with an impedance spectra analyzer (Nova) in 0.5 M Na2SO4 solution in the dark. Ag/AgCl and platinum wire were used as the reference and counter electrodes, respectively. All the electrochemical measurements were conducted using a twoelectrode glass cell setup consisting of a Nb2O5 film working electrode and Li foil counter/reference electrode. The electrolyte was 1 M LiClO4 in propylene carbonate (Panax Etec). All the test cells were assembled in an Ar-filled glovebox with moisture and oxygen levels below 1 ppm. Cyclic voltammetry (CV) measurements were performed using a potentiostat within a voltage window of 1.0−3.0 V (vs Li+/Li). Before the measurements, the fresh nanoporous electrodes were cycled five times at 10 mV s−1 to remove absorbed water or other impurities on the electrode surface, which could affect the total current in the CV curves.

EXPERIMENTAL SECTION

The Nb foils (0.5 mm thickness, 99.8%, Alfa Aesar) were prepared after a three-step (acetone−ethanol−deionized (DI) water) ultrasonic cleaning process; each step was performed for 10 min. Electrochemical anodization was conducted with a Pt counter electrode in 0.5 wt % NH4F (98%, Aldrich) and 4 vol % DI water containing ethylene glycol (98%, anhydrous, Aldrich). The voltage (50 V) and temperature (50 °C) were held constant for 1 h, and oxide films with a thickness of approximately 2 μm were produced. After the reaction was complete, the as-prepared anodic films were washed with ethanol solution under ultrasonication, and then dried with N2 gas. The amorphous Nb oxide films on Nb substrates were heated to 440 °C at a heating rate of 1 °C min−1 under each atmosphere (air, pure Ar, and 4% H2/96% Ar mixture) and maintained for 20 min to improve the crystallinity. After heating, the films were cooled down to room temperature naturally. The crystal structures of the Nb oxide films were identified by X-ray diffraction (XRD, Rigaku D/MAX-2500V). The morphology and microstructure of the prepared samples were examined by fieldemission scanning electron microscopy (FE-SEM, JEOL 7500) and high-resolution transmission electron microscopy (HR-TEM, JSM200FXII, JEOL) operating at 200 kV. X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe with Al Kα radiation) was used to

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RESULTS AND DISCUSSION The surface and cross-sectional FE-SEM images of the nanoporous and self-ordered Nb2O5 films prepared by anodizing Nb foils and then annealing under air (Figure 1a− c), Ar (Figure 1d−f), and H2 (Figure 1g−i) atmospheres are compared in Figure 1. As observed in the low-magnification surface images on the left side (Figure 1a,d,g), the films exhibit a hierarchical porous microstructure consisting of both macropores (>50 nm) and mesopores (2−50 nm). The sizes of the macropores decrease from nearly 1 μm (air-treated) to between 500 nm and 1 μm when treated with Ar or H2. The B


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Figure 2. Low- and high-magnification TEM and HRTEM images of Nb2O5 films subjected to different treatment atmospheres: (a−c) air and (d−f) H2.

influence the crystal structure as well as the morphology and microstructure of Nb2O5 films. Figure 3 shows the XRD patterns of Nb2O5 films annealed under various atmospheres. Annealing under all atmospheres

middle images (Figure 1b,e,h) show the nanoporous structure of the Nb2O5 film: air-annealed Nb2O5 film had a more dense structure, while Ar- and H2-treated Nb2O5 films exhibited relatively porous structures containing pores with diameters of less than 50 nm (Figure 1e,h). It should be noted that the annealing atmosphere can affect the microstructure of selforganized Nb2O5 films. The cross-sectional views of the films are displayed on the right side (Figure 1c,f,i). The thicknesses of all prepared Nb2O5 films were measured to be approximately 2 μm, and the films exhibited shapes resembling tree trunks. The microstructures of the porous thin films were further examined by TEM, and the results are presented in Figure 2. For the samples treated in air (Figure 2a−c) and H2 (Figure 2d−f), low- and high-magnification TEM and HR-TEM images were obtained. The low-magnification TEM images (Figure 2a,d, and Figure S1 with high-magnification FE-SEM images) confirm the hierarchical porous structure, comprising macropores and mesopores. This structure can provide a main pathway for electrolyte penetration through macropores with sub-micrometer sizes. Furthermore, the presence of numerous mesopores provides a large surface area for electrochemical charge transfer reactions. These unique features of the hierarchical pore structure should be beneficial for electrochemical energy storage devices, such as batteries and supercapacitors, which store electrical energy via surface electrochemical reactions. On the basis of the high-magnification TEM images (Figure 2b,e), the sizes of the mesopores in the air- and H2-treated Nb2O5 films were measured to be approximately 10−30 nm. Notably, the pore size and frame thickness of the H2-treated Nb2O5 film were somewhat smaller and thinner than those of the air-treated Nb2O5 film. Figure 2c,f shows the HR-TEM images of the frames of the Nb2O5 films. The air-treated Nb2O5 film shows a d-spacing of 3.14 Å, which was attributed to the (100) reflection from a pseudohexagonal crystal structure, whereas a d-spacing of 3.96 Å, corresponding to the (001) reflection of an orthorhombic structure, was observed for the H2-treated Nb2O5 film, which has well-defined crystallinity. On the basis of the SEM and TEM analyses, it can be concluded that the annealing atmosphere can significantly

Figure 3. XRD patterns of the air-, Ar-, and H2-treated Nb2O5 films.

transformed the amorphous as-grown films (Figure S2) into crystalline Nb2O5 phases. The air-annealed Nb2O5 film exhibited a pseudohexagonal structure (TT-Nb2O5, ICDDJCPDS No. 28-0317), and heating the films under an Ar or H2 atmosphere led to orthorhombic phases (T-Nb2O5, ICDDJCPDS No. 30-0873). Because the two patterns (TT-Nb2O5 and T-Nb2O5) are very similar, enlarged XRD patterns are displayed in Figure S3 to exactly identify the XRD patterns of the Nb2O5 films annealed under different atmospheres. The C


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Figure 4. XPS spectra of Nb 3d of the Nb2O5 films etched for 60 s: (a) air-, (b) Ar-, and (c) H2-treated samples, and (d) comparison.

Nb2O5 films are slightly shifted toward high binding energies compared with those of the air-treated Nb2O5 film. These shifts can be attributed to the differences in the crystal structure of Nb2O5, as confirmed by the XRD patterns (Figure 3) because the binding energies of Nb in different crystal structures should not be exactly the same, even if the stoichiometry is.5 In other words, the binding energies of Nb in further stable orthorhombic structures are slightly higher. Furthermore, as the annealing atmosphere changes from air to a neutral (Ar) or reducing atmospheres (H2), the shoulder at the low binding energy side grows remarkably, likely because of the increased abundance of oxygen vacancies. As reported in the literature, the quantitative analysis on each valence state of Nb (Table S1) may mislead the result because Ar-ion etching can affect the surface through a rapid reduction of metal species in oxide materials.43 Nevertheless, it is still concluded that the H2treated film has an increased amount of oxygen deficiencies compared to the other two films by considering the relatively higher shoulder at low binding energy region. The same tendency was also observed in the XPS results for 300 s etched samples (Figure S5). It is well-known that the oxygen deficiencies induced by H2 treatment increase the electronic conductivity of TiO2 systems.44 Given that the low electronic conductivity of self-ordered Nb2O5 films is considered to be a main obstacle for electrochemical Li storage kinetics, Ar or H2 treatment could be used to improve the Li storage kinetics of Nb2O5 films by increasing the electronic conductivity of materials. To address the effect of the heat-treatment atmosphere on the electronic conductivity and band structure of Nb2O5 films, each treated sample was subjected to Mott−Schottky analysis, and the plots are presented in Figure 5. The Mott−Schottky plots display the capacitance (C) of the space charge region as a function of the electrode potential and can be applied for semiconductor materials. Thus, these plots provide information on the carrier densities in terms of the gradient dV/d(1/C2)

two low-temperature forms of Nb2O5 (TT and T) had been considered as the same structure because they have similar XRD patterns and the TT phase does not always appear from pure components.37−39 However, the main difference can be observed from peak splitting of some reflections (Figure S3).1,38 This indicates that TT-Nb2O5 is a less crystalline form of T-Nb2O5, stabilized by impurities.1,38,39 In this study, as the annealing atmosphere varies from air (oxidizing) to Ar (neutral) and H2 (reducing), the resulting porous Nb2O5 films exhibit further stabilized forms. Recently, it was reported that the phase and crystallinity of Nb2O5 materials are closely related to the electrochemical Li storage properties. This relationship will be discussed with the electrochemical test results below. After observing the crystal structure modifications resulting from the treatment with Ar or H2, the chemical states of the Nb element were investigated by XPS. To determine the bulk states of the materials, XPS in-depth profiles were also measured while gradually increasing the Ar-ion etching time using a constant interval. These profiles revealed similar trends for all samples (see the Supporting Information, Figure S4). At the near surface, the Nb 3d peaks at approximately 209.6 and 206.9 eV can be assigned to Nb2O5.5,40−42 As the ion etching proceeds, the intensity of the XPS signal of the low binding energy site, corresponding to the Nb 3d5/2 peak, increases. The core-level Nb 3d peaks of all Nb2O5 films treated under each atmosphere after Ar-ion beam etching for 60 s, which corresponds to a depth of approximately 60 nm, are presented in Figure 4. The Nb 3d5/2 peaks were deconvoluted into four pitting curves, which can be assigned to the valence states of 0, +2, +4, and +5. As previously reported in the literature, these valence states correspond to Nb metal, NbO, NbO2, and Nb2O5, respectively.40−42 Figure 4d compares the Nb 3d spectra of the three films to elucidate the effect of the atmosphere used during heat treatment. The core-level Nb 3d peaks for Ar- and H2-treated D


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Figure 5. Mott−Schottky plots of the air-, Ar-, and H2-treated Nb2O5 films.

and the flat band potential (VFB), which is determined by extrapolating the linear part of the curve to the y-axis (1/C2) = 0 as follows Nd =

2 ⎡ dV ⎤ ⎥ ⎢ e0εε0 ⎣ d(1/C 2) ⎦

(1)

where e0 is the electronic charge, ε is the dielectric constant (77) of Nb2O5,1 ε0 is the permittivity of the vacuum, Nd is the donor density, and V is the applied voltage. All samples have positive slopes, confirming that they are n-type semiconductors. As shown in Figure 5a, the VFB of the air-treated Nb2O5 film is approximately −0.69 V (vs Ag/AgCl), which is in good agreement with the literature.45 Ar- and H2-treated Nb2O5 films exhibit VFB values of approximately −0.64 V (vs Ag/AgCl) (Figure 5b). From the magnitude of the slope, the calculated electron densities of the air-, Ar-, and H2-treated Nb2O5 films were measured to be 4.16 × 1016, 1.12 × 1018, and 3.31 × 1018 cm−3, respectively. Ar- and H2-treated Nb2O5 films showed electron densities that were improved by approximately 2 orders of magnitude relative to that of the air-treated Nb2O5 film because of the increased donor densities resulting from the increased abundance of oxygen vacancies. Furthermore, the H2treated Nb2O5 film exhibited a donor density that was 3 times higher than that of the Ar-treated Nb2O5 film. Therefore, the H2-treated Nb2O5 film should have the highest electronic conductivity among the three treatments. Figure 6 presents the CVs of hierarchically porous Nb2O5 electrodes recorded at scan rates of 0.1−10 mV s−1. In the cathodic part, the voltammetric current for all samples begins to increase at approximately 2.2 V vs Li+/Li, and relatively broad shapes are observed. In contrast, the anodic current responses exhibit peak shapes that vary according to the heat-treatment atmosphere. The anodic CV curves of the air-treated sample

Figure 6. Cyclic voltammograms of the Nb2O5 films at scan rates of 0.1−10 mV s−1 within a voltage window of 1.0−3.0 V vs Li+/Li.

include broad peaks (Figure 6a), which are more prominent for the films treated under Ar and H2 (Figure 6b,c, respectively). From the literature, it can be concluded that the cathodic and anodic currents result from the Li+ insertion and extraction reactions of the crystalline Nb2O5, respectively.5,46 These processes can be generally written as follows: x Li+ + x e− + Nb2 O5 ↔ LixNb2 O5

(2)

The mole fraction of inserted lithium (x) is 2−2.5 for crystalline Nb2O5 when the Nb oxide bulk structure is fully used for Li storage.5,12,46 For all samples, as the scan rate increases, the locations of the peaks in the anodic scans shift to higher potentials. Although the degrees of these shifts vary in the three films, this phenomenon is typically observed in CVs because more overpotential is needed for electrochemical reactions with high rates. To further explore the effect of the heat-treatment atmosphere and film morphology on the Li storage kinetics, the CVs were directly compared at each scan rate, as shown in Figure 7. At the lowest rate (0.1 mV s−1), the three electrodes exhibit similar broad peak shapes in both the cathodic and the E


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Figure 7. Comparisons of cyclic voltammograms at scan rates of (a) 0.1, (b) 0.5, (c) 2, and (d) 10 mV s−1.

Figure 8. (a) Real capacitance, (b) normalized capacitance, and (c) normalized anodic peak current (Li+ extraction) of the Nb2O5 electrodes as a function of the scan rate.

To precisely determine the electrochemical kinetics of the Nb2O5 electrodes, the CVs were quantitatively analyzed. Figure 8a,b shows the real and normalized capacitances of the Nb2O5 electrodes as a function of the scan rate. The capacitance values were calculated by integrating the anodic charge between 1.6 and 3.0 V vs Li+/Li. To observe the capacitance change as the scan rate increased, the capacitance was normalized against the same electrode at the slowest scan rate (0.1 mV s−1). At all scan rates exceeding 0.5 mV s−1, the H2-treated Nb2O5 film exhibited the best capacitance retention ratios. In addition, the retention of the Ar-treated Nb2O5 electrode was clearly superior to that of the air-treated electrode. This difference can be related to the material characterization results. According to SEM and XRD, both the H2- and Ar-treated samples have a porous structure consisting of relatively small pores and orthorhombic crystal structures. This porous structure may constitute a pathway for electrolyte penetration. Furthermore, it was recently reported that the orthorhombic structure of Nb2O5 is beneficial for Li storage capacity and rate capability compared with pseudohexagonal and amorphous structures.15

anodic curves (Figure 7a). The current density values for the air-treated Nb2O5 electrode are much higher than those of the other two electrodes. This difference might be attributable to the fact that the compact Nb2O5 layer at the interface between the anodized film and the Nb metal substrate is thicker than that in other samples because air annealing preferentially induces the formation of oxide layers during heat treatment. At the scan speed of 0.5 mV s−1, the profile shapes appear similar, although the extents of potential separation between the anodic and cathode peaks are different (Figure 7b). For the H2-treated sample, the peak potential separation was approximately 0.4 V, which was the smallest value. Thus, the Li+ insertion and extraction kinetics in the H2-treated Nb2O5 film were better than those in the other two films. As the scan rate increased to 2 mV s−1, the current density of the H2-treated film becomes the highest. At the highest scan rate (10 mV s−1), the H2treated sample exhibits the largest current density values and the smallest peak separation, implying that the sample has the best kinetics for Li+ insertion and extraction reactions. F


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these results, subjecting Nb2O5 films to H2 treatment modified their bulk morphologies and crystal structures, and improved their electronic conductivities by inducing oxygen deficiencies, thereby, greatly enhancing their electrochemical Li storage kinetics (Figure 9). To clearly understand this effect, further

Thus, the electrochemical kinetics of the H2- and Ar-treated Nb2O5 electrodes are better than those of the air-treated electrode. The difference between the H2-treated and Artreated electrodes can be seen in the XPS results and Mott− Schottky plots. The XPS results (Figure 4) show that the portion of low oxidation states in the Nb profile increases after Ar and H2 treatment compared with air treatment, and that of the H2-treated film is higher than that of the Ar-treated film. The low oxidation state of Nb controls the number of oxygen deficiencies in Nb2O5 films. These results are in good agreement with those of the Mott−Schottky analysis. As described above, Ar- and H2-treated Nb2O5 films exhibited greatly increased donor densities compared to the air-treated film, with the H2-treated Nb2O5 film showing the highest value. As a result, the electronic conductivity of the H2-treated Nb2O5 film should be the highest. The H2-treated Nb2O5 electrode also had the best capacitance retention, as shown by Figure 8b. The low electronic conductivity of the air-treated Nb2O5 electrode could be a main reason underlying this film’s poor rate capability.36 Thus, the H2 treatment modified the morphology and crystal structure of the Nb2O5 film, improving its fast lithium storage capability. Furthermore, induced oxygen deficiencies (vacancies) increased the electronic conductivity of materials, and thus, the rate performance of the H2-treated Nb2O5 electrode was greatly enhanced. To examine the Li+ storage mechanism in the Nb2O5 electrodes, the anodic peak current values corresponding to Li+ extraction were evaluated while increasing the scan rate. The normalized peak current densities as a function of the scan rate are shown in Figure 8c (measured from Figure 6). The voltammetric current can be generally expressed as a function of the scan rate (υ) as follows47,48

I = aυ b

Figure 9. Schematic illustration for the effect of H2 treatment on nanoporous Nb2O5 films.

investigation of the material properties of H2-treated Nb2O5 is required. Additionally, this treatment should be applied to other oxide materials.

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CONCLUSIONS Self-ordered Nb2O5 films were prepared by anodizing Nb foils and subsequently treating them in a H2 atmosphere. SEM and HRTEM revealed that the Nb2O5 film had a hierarchical porous microstructure consisting of both macropores and mesopores and that H2 treatment can affect the morphology of the porous Nb2O5 film. XRD showed that the bulk crystal structure of the Nb2O5 film can be changed to an orthorhombic structure by the H2 treatment, whereas the air-treated film retains a pseudohexagonal structure. In addition, this treatment can induce oxygen deficiencies in the film, as confirmed by XPS. Mott−Schottky analysis was performed and confirmed that the electronic conductivity of the material was greatly enhanced by the oxygen deficiencies. Thus, the electrochemical Li storage kinetics of porous Nb2O5 films can be significantly improved by H2 treatment. Therefore, this method should be useful for enhancing the Li storage kinetics of nanoporous Nb2O5 films and could be applied to other porous metal oxide films.

(3)

where a is a prefactor and b is a parameter that depends on the Li+ storage mechanism.49−51 When the current is controlled by solid-state cation diffusion in the bulk of the material, the b value can be 0.5. If the total current is determined by surface charge transfer, the current depends linearly on the scan rate (b = 1).52 For the limiting cases, ionic transport in the electrolyte and electronic transport in electrode should be no problem. When the b value was calculated using eq 3 between 0.1 and 0.5 mV s−1, b was 0.92 for both the Ar- and H2-treated electrodes and 0.72 for the air-treated electrode. In this low-scan rate region, the value of 0.92 for Ar- and H2-treated electrodes indicates that the charge storage is mostly controlled by surface reactions. In contrast, the Li+ storage in the air-treated electrode is dependent on cation diffusion in the bulk lattice and the surface reactions. In the midscan rate region (0.5 and 2 mV s−1), the H2treated electrode maintained its linear dependence and b values of 0.92, indicating that the Li storage reaction is fast and not limited by other factors. In contrast, b decreased to 0.88 for the Ar-treated electrode and further decreased to 0.51 for the airtreated sample, for which the kinetics were nearly completely controlled by Li+ diffusion. In the high-scan rate region between 2 and 10 mV s−1, the normalized peak current in all three electrodes did not retain its linear dependence (log I vs log υ). In the air-treated electrode, b was 0.25, which is less than 0.5. In this case, another limiting factor, the low electronic conductivity of Nb2O5, is also present, as reported previously.36 In the H2-treated electrode, however, b was 0.70 because the electronic conductivity was not a main limiting factor. From

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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/acs.chemmater.5b04845. Additional SEM and TEM images of Nb2O5 films subjected to different treatment atmospheres, XRD patterns of as-grown and annealed Nb2O5 films, XPS in-depth profiles for Nb 3d of the Nb2O5 films, and table of quantitative analysis results calculated from XPS spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.H.K.). *E-mail: [email protected] (J.-H.K.). G


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Chemistry of Materials Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2009-0093814). This work was also supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP, 2015R1A5A7037615).

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