Electrochemical Synthesis of Binary and Ternary Niobium-Containing

Article pubs.acs.org/Langmuir

Electrochemical Synthesis of Binary and Ternary Niobium-Containing Oxide Electrodes Using the p‑Benzoquinone/Hydroquinone Redox Couple Christopher M. Papa, Anthony J. Cesnik, Taylor C. Evans, and Kyoung-Shin Choi* Department of Chemistry, University of WisconsinMadison, Madison, Wisconsin 53706, United States

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 26, 2015 | http://pubs.acs.org Publication Date (Web): August 21, 2015 | doi: 10.1021/acs.langmuir.5b01665

S Supporting Information *

ABSTRACT: New electrochemical synthesis methods have been developed to obtain layered potassium niobates, KNb3O8 and K4Nb6O17, and perovskite-type KNbO3 as film-type electrodes. The electrodes were synthesized from aqueous solutions using the redox chemistry of p-benzoquinone and hydroquinone to change the local pH at the working electrode to trigger deposition of desired phases. In particular, the utilization of electrochemically generated acid via the oxidation of hydroquinone for inorganic film deposition was first demonstrated in this study. The layered potassium niobates could be converted to (H3O)Nb3O8 and (H3O)4Nb6O17 by cationic exchange, which, in turn, could be converted to Nb2O5 by heat treatment. The versatility of the new deposition method was further demonstrated for the formation of CuNb2O6 and AgNbO3, which were prepared by the deposition of KNb3O8 and transition metal oxides, followed by thermal and chemical treatments. Considering the lack of solution-based synthesis methods for Nb-based oxide films, the methods reported in this study will contribute greatly to studies involving the synthesis and applications of Nb-based oxide electrodes.

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catalysis applications.19−22 Because Nb2O5 is stable at all pH conditions,23 it is also a promising candidate material to serve as a passivation layer for various electrochemically and photoelectrochemically active electrodes. The versatility of our newly developed electrochemical synthesis conditions can be further extended to the production of ternary niobium oxides containing transition metals. Synthesis of CuNb2O6 and AgNbO3 electrodes will be demonstrated as examples in this study, which shows the reduction of band gap energy by the incorporation of transition metals. Considering that solution-based synthesis methods for Nb-based oxide films have been quite limited as a result of the insolubility of Nb5+ species in aqueous solutions, the methods reported in this study will contribute greatly to studies involving the synthesis and applications of Nb-containing oxide electrodes.

INTRODUCTION KNb3O8 and K4Nb6O17 are two of the best-known, moststudied layered niobate phases (panels a and b of Figure 1).1−8 Owing to their layered structures, they have been investigated for use in heterogeneous catalysis, sensing, molecular sieves, and initial building blocks to form nanocomposite materials.1−8 KNbO3, another important potassium niobate, has a perovskitetype structure (Figure 1c), with notable piezoelectric, ferroelectric, and nonlinear optical properties.9−13 It has been studied for nonlinear optics, optical wave guiding, photocatalysis, and biocompatible small-molecule sensing.9−13 The most common synthesis method used to prepare these potassium niobates is a high-temperature solid-state reaction, but hydrothermal, sol−gel, and flux synthesis methods have also been reported.2−4,7−9,11 While some of the applications of these compounds require them to be processed as electrodes, synthesis methods that can grow them directly on the conducting substrate as film-type electrodes have been extremely limited to date. In this study, we report electrochemical synthesis methods that can produce KNb3O8, K4Nb6O17, and KNbO3 as film-type electrodes on the conducting substrates. The resulting KNb3O8 and K4Nb6O17 electrodes can also be converted to (H3O)Nb3O8 and (H3O)4Nb6O17 electrodes, respectively, via ion exchange of K + with H 3 O + , owing to their layered structures.6,14,15 (H3O)Nb3O8 and (H3O)4Nb6O17 have been investigated as solid acid catalysts and photocatalysts.16−18 When (H3O)Nb3O8 and (H3O)4Nb6O17 electrodes are annealed, Nb2O5 electrodes can be additionally obtained additionally. Nb2O5 can be used for energy storage and © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Materials. NH4NbO(C2O4)2 (99.99%, Aldrich), p-benzoquinone (98%, Sigma-Aldrich), hydroquinone (99%, Acros), potassium hydroxide (≥85%, Sigma-Aldrich), Cu(NO3)2·3H2O (99%, Acros), AgNO3 (≥99.9%, Alfa Aesar), KNO3 (99%, Alfa Aesar), nitric acid (69−70%, BDH), sulfuric acid (95.0−98.0%, Sigma-Aldrich), and hydrochloric acid (37%, Sigma-Aldrich) were used without further modification. Deionized (DI) water from a Barnstead E-pure Received: May 8, 2015 Revised: August 13, 2015

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DOI: 10.1021/acs.langmuir.5b01665 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. Crystal structures of (a) KNb3O8, (b) K4Nb6O17, and (c) KNbO3. The green polyhedra represent NbO6 octahedral units, and the purple spheres represent K+ ions. good coverage and adhesion. After deposition, the films were rinsed with DI water and dried by flowing air over them. The films were annealed for 2 h at 600 °C in air (ramp rate = 2.4 °C/min). Synthesis of KNbO3 by Anodic Electrodeposition. The plating solution for anodically deposited potassium niobium oxide films was prepared by heating a 150 mM solution of NH4NbO(C2O4)2 in DI water to 60 °C, raising the pH to 12.0 by adding KOH, and dissolving the required amount of hydroquinone to achieve 100 mM in solution. Finally, the pH of the solution was decreased to 10.1 by adding HNO3. This is again to bring the pH of the solution near the precipitation point to minimize the current required to decrease the pH to trigger the precipitation. The anodic deposition was carried out at E = +2.3 V versus Ag/AgCl for 3 min by oxidizing hydroquinone to pbenzoquinone, which generated H+ ions, resulting in precipitation of KNbO3 on the WE surface. A typical deposition passed ca. 4.4 C/cm2. When a more negative deposition potential was used (E < +2.3 V versus Ag/AgCl), the adhesion, uniformity, and surface area of asdeposited films decreased. The films were carefully rinsed by dipping in 60 °C DI water and dried by gently flowing air over them. The films were annealed for 2 h at 600 °C in air (ramp rate = 2.4 °C/min). Cationic Exchange of KNb3O8 and K4Nb6O17 To Form Nb2O5. Previously annealed films of KNb3O8 and K4Nb6O17 were immersed in 1.4 M HNO3 for 3 h with stirring to exchange H3O+ for K+ throughout the sample. The resulting (H3O)Nb3O8 and (H3O)4Nb6O17 films were then rinsed with DI water and dried by flowing air over them. The films were converted to Nb2O5 by annealing for 2 h at 600 °C in air (ramp rate = 2.4 °C/min). Synthesis of CuNb2O6. A plating solution was prepared by adding 1.25 mL of a 200 mM Cu(NO3)2 solution to 50 mL of a 50 mM NH4NbO(C2O4)2 solution after the pH was raised above 4 by adding KOH. Then, p-benzoquinone equivalent to 20 mM was added to the solution, followed by stirring and sonication. The final pH of the solution was adjusted to be ∼4.6 by adding KOH. Cathodic co-deposition of Cu2O and KNb3O8 was carried out by passing 0.61 C/cm2 at E = −0.7 V versus Ag/AgCl. The films were rinsed with DI water and dried by flowing air over it. During heat treatment for 1 h at 650 °C in air (ramp rate = 2.6 °C/min), Cu2O and KNb3O8 reacted to form CuNb2O6, while the K ions in KNb3O8 appeared to form amorphous K-containing impurities. The Kcontaining impurities and excess, unreacted Cu2O were removed by acid treatment in 0.1 M HNO3 for 2 h with stirring. Synthesis of AgNbO3. The plating solution was prepared using the same procedure described for the synthesis of CuNb2O6. The only difference was that the volume of the 200 mM Cu(NO3)2 solution added was increased from 1.25 to 2 mL. Cathodic co-deposition of Cu2O and KNb3O8 was carried out by passing 0.61 C/cm2 at E = −0.7 V versus Ag/AgCl. After the film was rinsed with DI water, Cu2O was then galvanically displaced with Ag by dipping the film in a 20 mM AgNO3 solution for 5 min. The film was rinsed with DI water and

purification system (resistivity of 18 MΩ cm) was used to prepare all aqueous solutions. Electrochemical Synthesis Cell. All films were electrodeposited in an undivided cell using a multi-channel potentiostat (VMP2, Princeton Applied Research). A standard three-electrode cell composed of a fluorine-doped tin oxide (FTO) working electrode (WE), a Ag/AgCl (4 M KCl) reference electrode, and a platinum counter electrode was used. The platinum counter electrode was prepared by sputter coating 20 nm of Ti, followed by 100 nm of Pt, on a clean glass substrate. Synthesis of KNb3O8, K4Nb6O17, and KNbO3 by Cathodic Electrodeposition. For cathodic deposition of KNb3O8, K4Nb6O17, and KNbO3 films, a 50 mM NH4NbO(C2O4)2 solution was commonly used as the main plating solution. However, the concentration of K+ ions in the plating solution was varied to obtain three different K/Nb ratios in the deposited films. For the preparation of KNb3O8, the pH of the 50 mM NH4NbO(C2O4)2 solution was raised to 4.8 by adding KOH. The K+ ions introduced in this process (∼80 mM) were sufficient to deposit an amorphous KNb3O8 film. For the preparation of K4Nb6O17, the concentration of K+ was raised to 0.20 M by adding KNO3. Then, the pH of the solution was raised to 3.4 by adding KOH. For the preparation of KNbO3, the concentration of K+ was raised to 1.25 M by the addition of KNO3 and then the plating solution pH was raised to 2.8 by adding KOH. As discussed below, the deposition of potassium niobate films is triggered by a local pH increase on the WE, which decreases the solubility of the potassium niobate species in solution. To make the deposition efficient by minimizing the amount of OH− required to trigger the deposition, the pH of the solution was raised to a pH very near the precipitation pH. Because each solution with a different composition has a different precipitation pH, the pH of the solution was adjusted to a different value. All pH values were determined with an Accumet AR-15 pH meter (Fisher) and an accuTupH electrode with automatic temperature compensation. For all cathodically deposited potassium niobium oxide films, the required amount of p-benzoquinone to achieve 20 mM in solution was added and stirred vigorously, followed by sonication to ensure complete dissolution of p-benzoquinone. The cathodic deposition was carried out at E = −1.2 V versus Ag/AgCl, causing the reduction of pbenzoquinone to hydroquinone, which generated OH− ions, resulting in precipitation of potassium niobate on the WE surface. A typical deposition passed 0.61 C/cm2. Before the deposition potential was chosen, linear sweep voltammograms (LSVs) were recorded in each plating solution by sweeping the potential from the open circuit potential to the negative direction to identify the onset potential of benzoquinone reduction (Figure S1 of the Supporting Information). Then, a series of deposition potentials more negative than the onset of benzoquinone reduction were used for film depositions, and the qualities of the resulting films were examined. The potentials reported in this study were the optimum potentials that resulted in films with B


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Langmuir dried by flowing air over it. AgNbO3, K-containing impurities, and excess crystalline Ag metal were formed by heat treatment for 2 h at 600 °C in air (ramp rate = 2.4 °C/min). The K-containing impurities and excess Ag were removed by rinsing with concentrated HNO3, followed by rinsing with DI water. Characterization. The surface morphology and elemental composition of the electrodeposited films were characterized by scanning electron microscopy (SEM, LEO Supra55 VP) and energydispersive X-ray spectroscopy (EDS, Noran System Six, ThermoFisher). The crystal structures and impurities of each film were investigated by powder X-ray diffraction (XRD, D8 Discover, Bruker, 298 K, Ni-filtered Cu Kα radiation, λ = 1.5418 Å). Ultraviolet−visible (UV−vis) absorption spectra were obtained by placing the sample electrode in the center of an integrating sphere to measure all light reflected and transmitted to accurately assess the absorbance (Cary 5000 UV−vis−NIR spectrophotometer, Agilent). Electrochemical and Photoelectrochemical Characterization. Photocurrent measurements for CuNb2O6 and AgNbO3 were carried out using a SP-200 potentiostat/EIS (Bio-Logic) and simulated solar illumination obtained by passing light from a 300 W Xe arc lamp through neutral density filters, an AM 1.5G filter, and a water filter into an optical fiber. Illumination through the FTO was used, which provides a slightly enhanced photocurrent signal compared to illumination through the sample. The light power density was calibrated to 100 mW/cm2 (1 sun) before passing through FTO using both a thermopile detector (International Light) and a National Renewable Energy Laboratory (NREL)-certified reference cell (Photo Emission Tech, Inc.). All oxide electrodes were masked with epoxy resin to make the exposed geometrical area (0.02 cm2) smaller than the illuminated area (0.06 cm2). An undivided three-electrode cell composed of a WE (CuNb2O6 or AgNbO3), a Pt counter electrode, and a Ag/AgCl (4 M KCl) reference was used. All photocurrent measurements were made while sweeping the potential with a scan rate of 10 mV/s in a 0.1 M sodium hydroxide solution (pH 12.8). Although all measurements were carried out using a Ag/AgCl (4 M KCl) reference electrode, all results in this work are presented against the reversible hydrogen electrode (RHE) for ease of comparison to other reports that used electrolytes with different pH conditions. The conversion between potentials versus Ag/AgCl and versus RHE is performed using the equation below.

The as-deposited films were amorphous but could be converted to crystalline KNb3O8, K4Nb6O17, and KNbO3 electrodes after annealing for 2 h at 600 °C in the air. The XRD patterns of the resulting films matched very well with those reported in the literature, and their phases could be unambiguously identified (Figure 2). It should be noted that

Figure 2. XRD patterns for cathodically deposited (a) KNb3O8 (PDF number: 01-075-2182), (b) K4Nb6O17 (PDF number: 01-076-0977), and (c) KNbO3 (PDF number: 01-077-1098) films after annealing for 2 h at 600 °C in air. The peaks from the FTO substrate are denoted by asterisks.

the XRD pattern of K4Nb6O17 shown in Figure 2b is for its anhydrous phase obtained immediately after annealing. However, because of the facility of interlayer hydration of this phase, new peaks belonging to the hydrated phase appeared over time when the film was exposed to the air, even at room temperature.26 For example, a very tiny peak, shown in Figure 2b at 9.32°, is the strongest peak of hydrated K4Nb6O17,6,14,26 which had already started to form after the annealing process before the XRD measurement. SEM images of KNb3O8, K4Nb6O17, and KNbO3 electrodes before and after annealing are shown in Figure 3. While KNb3O8 was deposited with a rough surface, creating a high surface area (Figure 3a), K4Nb6O17 was deposited with a rather featureless surface morphology (Figure 3c). KNbO3 was also deposited as a featureless film, but once the FTO substrate was completely covered, it was deposited as spherical nanoparticles (Figure 3e). However, the featureless layer and the nanoparticles have the same composition. All of the electrodes retained their original morphologies after annealing (panels b, d, and f of Figure 3). Higher magnification and side-view SEM images of these and other films discussed later are included in Figures S2 and S3 of the Supporting Information. We attempted to deposit Nb2O5 by not introducing K+ ions in the plating solution while using the same deposition mechanism. In this case, necessary pH adjustment was made using NH3 solution instead of KOH. However, it was discovered that no films could be deposited without the presence of K+ in the NH4NbO(C2O4)2 solution.

E (versus RHE) = E (versus Ag/AgCl) + EAg/AgCl (reference) + 0.0591 V × pH

(EAg/AgCl (reference) = 0.1976 V versus NHE at 25 °C)

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RESULTS AND DISCUSSION

Cathodic Deposition of KNb3O8, K4Nb6O17, and KNbO3 and Their Conversion. NH4NbO(C2O4)2, which is the only commercially available water-soluble Nb precursor, was used as the Nb source for the preparation of the plating solution. This species is soluble in an acidic aqueous medium, but when the pH increases and K+ ions are available, Nb5+ and K+ ions coprecipitate to form amorphous potassium niobate phases. When the concentration of K+ was varied in solution, three different phases, KNb3O8, K4Nb6O17, and KNbO3, could be obtained. To grow KNb3O8, K4Nb6O17, and KNbO3 on the WE surface without triggering bulk precipitation in solution, only the local pH on the WE was raised by electrochemically generating base on the electrode surface. The reduction of pbenzoquinone to hydroquinone was used to increase the pH on the WE (eq 1).24,25 C


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Figure 4. XRD patterns for (a) KNb3O8, (b) acid-treated KNb3O8 [=(H3O)Nb3O8] (JCPDS number: 44-672), and (c) R−Nb2O5 (PDF number: 01-074-0312) obtained by annealing (H3O)Nb3O8. Peaks from the FTO substrate are denoted by asterisks.

Figure 3. SEM images of (a) as-deposited and (b) annealed KNb3O8, (c) as-deposited and (d) annealed K4Nb6O17, and (e) as-deposited and (f) annealed KNbO3.

The layered potassium niobates, KNb3O8 and K4Nb6O17 (panels a and b of Figure 1), are known to undergo cationic exchange to replace K+ in the bulk structure.6,14,15 Therefore, we attempted to replace K+ with H3O+, postulating that annealing the resulting (H3O)Nb3O8 and (H3O)4Nb6O17 electrodes may provide a route to obtain Nb2O5 electrodes. In addition, (H3O)Nb3O8 and (H3O)4Nb6O17 electrodes can also be useful for solid acid catalysis and photocatalysis.16−18 The cationic exchange was performed by immersing the annealed KNb3O8 and K4Nb6O17 electrodes in 1.4 M HNO3 for 3 h with stirring. Elemental analysis by EDS showed that the amount of K+ ions remaining in the resulting electrodes was negligible. Also, the acid-treated samples showed clear shifts of XRD peaks to lower 2θ values. For example, the largest peak, (020), for the KNb3O8 system shifted from 8.35° to 7.75°, while the largest peak, (400), for the K4Nb6O17 system shifted from 10.64° to 9.32°. This suggests an increase in the unit cell parameters, consistent with the replacement of K+ ions by larger H3O+ ions6,14,15 (Figures 4 and 5). Judging from the peak intensities and shapes, (H3O)Nb3O8 appeared to retain the high crystallinity of KNb3O8 (Figure 4), while (H3O)4Nb6O17 became less crystalline and only one main peak, (400), remained with a broadened peak shape (Figure 5). When (H3O)Nb3O8 and (H3O)4Nb6O17 films were annealed for 2 h at 600 °C in air, they were converted to R−Nb2O5 (space group: A2/m) and T−Nb2O5 (space group: Pbam), respectively (Figure 4c and Figure 5c). The formation of R−

Figure 5. XRD patterns for (a) K4Nb6O17, (b) acid-treated K4Nb6O17 [=(H3O)4Nb6O17], and (c) T−Nb2O5 (JCPDS number: 30-0873) obtained by annealing (H3O)4Nb6O17. Peaks from the FTO substrate are denoted by asterisks.

Nb2O5 from (H3O)Nb3O8 at elevated temperatures has been previously reported,15 while T−Nb2O5 is one of the more thermodynamically stable known forms of Nb2O5.27 Our results demonstrate that electrochemical deposition of layered potassium niobates followed by ion exchange of K+ with H3O+ can provide indirect routes to prepare Nb2O5 films with two different structures, which are difficult to directly prepare as films. The XRD peaks of both Nb2O5 films are weak and broad, indicating the nanocrystallinity of these films. The SEM images of R−Nb2O5 and T−Nb2O5 electrodes as well as (H3O)Nb3O8 and (H3O)4Nb6O17 are shown in Figure 6. All of them reserved the original morphologies of corresponding as-deposited potassium niobate electrodes. Anodic Deposition of KNbO3. The aqueous NH4NbO(C2O4)2 solution exhibits amphoteric behavior in the presence of K+, where the potassium niobate precipitate obtained by raising the pH dissolves back into solution when the pH is further increased to around pH 12. This behavior does not occur when K+ is replaced with NH4+, Li+, or Na+. This amphoteric behavior suggests that, if the pH of the plating D


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Figure 6. SEM images of (a) (H3O)Nb3O8, (b) R−Nb2O5, (c) (H3O)4Nb6O17, and (d) T−Nb2O5.

solution is higher than 12, electrochemical deposition of potassium niobates should also be possible by the electrochemical generation of acid. The most common electrochemical route to generate H+ ions is water oxidation that involves O2 evolution (eq 2).23 2H 2O → O2 + 4H+ + 4e−

E° = 1.23 V

(2)

However, concomitant O2 evolution during film deposition can result in poor film adhesion and non-uniform film morphology. Oxidation of ascorbate dianion was recently demonstrated as an alternative electrochemical route to generate H+, which does not involve O2 evolution.28 In this study, the reverse reaction of benzoquinone reduction, oxidation of hydroquinone (eq 3),25 was for the first time employed to lower the local pH at the WE. As hydroquinone was oxidized to p-benzoquinone, it consumed OH− and lowered the local pH on the WE, resulting in the deposition of KNbO3 films.

Figure 7. (a) LSVs obtained in the plating solution prepared for anodic deposition with (red) and without (black) 100 mM hydroquinone (scan rate = 10 mV/s). (b) XRD pattern of anodically deposited KNbO3 after annealing for 2 h at 600 °C in air. FTO substrate peaks denoted by asterisks. SEM images of anodically deposited KNbO3 (c) before and (d) after annealing.

contained 20 mM p-benzoquinone and the average deposition current density was 1.47 mA/cm2, while the plating solution used for anodic deposition contained 100 mM hydroquinone and the average deposition current density was 19.7 mA/cm2. In general, fast deposition results in films with rougher morphologies and higher surface areas by creating diffusionlimited growth conditions. From cathodic deposition, three potassium niobate phases (KNb3O8, K4Nb6O17, and KNbO3) were obtained by varying the amount of K+ present in the plating solution. However, only KNbO3 was obtained by anodic deposition because of the high concentration of K+ in the anodic plating solution necessary for raising the pH from ∼1 to ∼12 with KOH. Attempts to anodically deposit KNb3O8 and K4Nb6O17 by increasing the concentration of NH4NbO(C2O4)2 to achieve desired K/Nb ratios led to precipitation in the bulk solution. Synthesis of CuNb2O6 and AgNbO3. The cathodic deposition conditions developed to obtain Nb-containing thin

A comparison of LSVs for the plating solution with and without hydroquinone (Figure 7a) shows the earlier onset for hydroquinone oxidation relative to the oxidation of any other species present in the system (e.g., oxidation of water or oxalate ions). The as-deposited film was amorphous, but after annealing at 600 °C, it was converted to crystalline KNbO3 (Figure 7b). In comparison to cathodically prepared KNbO3, anodically prepared KNbO3 showed a much rougher surface morphology and, therefore, a higher surface area, which can be advantageous for various applications (Figure 7c). This is mainly because the solubility of hydroquinone is much higher than that of pbenzoquinone in an aqueous solution, and therefore, anodic deposition could achieve a much higher deposition current. For example, the plating solution used for cathodic deposition E


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Langmuir films can be combined with co-deposition and post-deposition processes to obtain a wide variety of ternary Nb-containing oxides as thin-film electrodes. Two example cases are highlighted here. To synthesize CuNb2O6, KNb3O8 and Cu2O were codeposited using the same deposition condition and plating solution used for cathodic KNb3O8 deposition. The only difference was the addition of 5 mM Cu(NO3)2 to the plating solution as the Cu source. The potential applied to reduce pbenzoquinone to hydroquinone was sufficient to reduce Cu2+ ions to Cu+ ions, resulting in the co-deposition of Cu2O (Cu2+ + H2O + e− → Cu2O + 2H+). The as-deposited film bears a morphological resemblance to a film of only KNb3O8 (Figure 8a). XRD of the as-deposited film (Figure 8c) shows only peaks

The resulting CuNb2O6 electrode is yellowish green in color. The color of the film matches the literature description of the color of powders of the monoclinic polymorph.29,30 The SEM image shows that the morphology of the CuNb2O6 particles is different from that of the as-deposited film, but it retained surface roughness and high surface area (Figure 8b) . The synthesis of AgNbO3 followed a similar pathway; Cu2O and KNb3O8 were co-deposited, and Cu2O was galvanically replaced with Ag by submerging the film in a 20 mM AgNO3 solution for 5 min (eq 4).23 When a KNb3O8/Ag composite electrode was annealed for 2 h at 600 °C in air, AgNbO3 was formed. The remaining Ag- and K-containing phases were removed by rinsing with concentrated HNO3, followed by DI water.


Cu 2O + 2Ag + + 2H+ → H 2O + 2Cu 2 + + 2Ag E° = 0.596 V

(4)

This process was used in lieu of co-depositing Ag metal and KNb3O8 from the plating solution because Ag+ ions are insoluble in the plating solution used in this study. Because the Ag/Nb ratio in AgNbO3 is higher than the Cu/Nb ratio in CuNb2O6, the concentration of Cu(NO3)2 in the plating solution was raised to 8 mM to ensure sufficient deposition of Cu2O in the precursor film to form AgNbO3. SEM studies show that, while the morphologies of the asdeposited films were similar to that of the as-deposited films for CuNb2O6 (Figure 9a), the morphology of the AgNbO3 films after galvanic displacement (Figure 9b) and annealing (panels c and d of Figure 9) was noticeably different from that of CuNb2O6. XRD confirmed the presence of AgNbO3 after annealing and the removal of excess Ag by treatment with acid (Figure 9e). EDS before and after the acid treatment step confirmed the removal of K, the absence of unreacted Cu, and the desired 1:1 Ag/Nb atomic ratio. Bandgap, Photoactivity, and Chemical Stability. All potassium niobates (KNb3O8, K4Nb6O17, and KNbO3), cationexchanged phases, and Nb2O5 phases were white, suggesting that they cannot absorb visible light as a result of their wide bandgaps. The direct band gap energies of these compounds were estimated using Tauc plots obtained from UV−vis absorption spectra. The bandgaps of KNb 3 O 8 and H3ONb3O8 were estimated to be Eg = ca. 3.50 eV (Figure 10a), while the bandgaps of K4Nb6O17 and (H3O)4Nb6O17 were estimated to be Eg = ca. 3.54 eV (Figure 10b). These values are consistent with previously reported values in the literature.7 All four layered compounds commonly exhibit a shoulder feature around 380 nm. KNbO3 made via either cathodic or anodic deposition has a band gap energy (Eg) of ca. 3.29 eV (Figure 10c), in good agreement with the literature value of 3.30 eV.31 The two forms of Nb2O5 made from the two different acid-treated layered potassium niobates have band gap energies of ca. 3.35 eV (Figure 10d), in good agreement with the literature value of 3.40 eV.32 CuNb2O6 shows a yellowish green color, suggesting that its bandgap is in the visible region. On the other hand, the color of AgNbO3 is white, indicating that it has a wider bandgap. The bandgap energies of CuNb2O6 and AgNbO3 were estimated to be 2.7 and 3.0 eV, respectively, using UV−vis absorption spectra (Figure 10e). CuNb2O6 and AgNbO3 have been previously studied as photocatalysts.33−38 Therefore, we investigated the photoactivity of CuNb2O6 and AgNbO3 electrodes by obtaining

Figure 8. SEM images of (a) as-deposited KNb3O8 and Cu2O on FTO before annealing and (b) pure crystalline CuNb2O6 after annealing and acid treatment. (c) XRD patterns for co-deposited KNb3O8 and Cu2O (i) as-deposited and (ii) after annealing. (iii) CuNb2O6 (PDF number: 01-083-0369) obtained after acid treatment (■, Cu2O; ●, CuNb2O6; and ∗, FTO substrate).

corresponding to crystalline Cu2O because as-deposited KNb3O8 is amorphous. Heat treatment for 1 h at 650 °C in air leads to the formation of CuNb2O6, indicating that CuNb2O6 is thermodynamically more stable than KNb3O8. Upon the formation of CuNb2O6, the K ions originally present in the as-deposited KNb3O8 film appear to form other amorphous potassium oxide or potassium copper oxide phases. In addition to the peaks of CuNb2O6, the XRD data also show the presence of Cu2O (Figure 8c). This is because the composition of the plating solution was intentionally formulated to deposit more Cu2O than necessary to form CuNb2O6. The presence of excess Cu2O ensures more uniform formation of CuNb2O6 throughout the film, and unreacted Cu2O as well as potassium-containing phases can be easily removed by dissolution in 0.1 M HNO3 for 2 h with stirring, where CuNb2O6 is stable. EDS measurements before and after the acid treatment step confirmed both the removal of K and the final desired 1:2 Cu/Nb atomic ratio. The XRD study also shows only the peaks that belong to CuNb2O6 after the acid treatment (Figure 8c). F


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Figure 9. SEM images of as-deposited KNb3O8 and Cu2O on FTO (a) before and (b) after galvanic displacement of Cu by Ag and AgNbO3 after annealing (c) before and (d) after acid treatment to remove K and excess Ag. (e) XRD patterns for co-deposited KNb3O8 and Cu2O on FTO (i) before and (ii) after galvanic displacement of Cu by Ag, (iii) AgNbO3 (PDF number: 01-073-8074) after annealing and before acid treatment, and (iv) AgNbO3 after acid treatment to remove K and excess Ag (□, Cu2O; ■, Ag; ●, AgNbO3; and ∗, FTO substrate).

Figure 10. UV−vis absorbance spectra for (a) KNb3O8 (black) and H3ONb3O8 (red), (b) K4Nb6O17 (black) and (H3O)4Nb6O17 (red), (c) cathodic KNbO3 (black) and anodic KNbO3 (red), and (d) R− Nb2O5 (black) and T−Nb2O5 (red). (e) UV−vis absorption spectra of CuNb2O6 (black) and AgNbO3 (red). J−V plots of (f) CuNb2O6 and (g) AgNbO3 electrodes under AM 1.5G (100 mW/cm2) illumination in 0.1 M NaOH (pH 12.8) (scan rate = 10 mV/s).

photocurrent−potential (J−V) plots for water reduction or oxidation in 0.1 M NaOH solution. CuNb2O6 films show a ptype behavior by generating a cathodic photocurrent, while AgNbO3 films show an n-type behavior by generating an anodic photocurrent (panels f and g of Figure 10). The photocurrents generated by both CuNb2O6 and AgNbO3 under AM 1.5G illumination were not considerable as a result of their wide bandgap energies, limiting photon absorption, and also possibly as a result of severe surface electron−hole recombination. Because the photocurrents in panels f and g of Figure 10 were measured with chopped light, dark currents of CuNb2O6 and AgNbO3 are also displayed in this figure. The dark current shows that both compounds are not reductively stable and cathodic dark current initiates between 0.6 and 0.7 V versus RHE. This indicates that the facile reduction of Cu2+ to Cu+ in CuNb2O6 and Ag+ to Ag0 in AgNbO3 makes these compounds electrochemically unstable, which will limit the photoelectrochemical application of these compounds.

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SUMMARY A novel electrochemical synthesis method was developed to obtain layered potassium niobates, KNb3O8 and K4Nb6O17, and perovskite-type KNbO3 as film-type electrodes. The electrodes were synthesized from aqueous acidic solutions using the reduction of p-benzoquinone, which generates OH− at the WE, inducing the precipitation of desired phases. KNbO3 could also be produced from aqueous basic solutions by oxidation of hydroquinone, which generates H+, using the amphoteric nature of the niobate precursor used in this study. This was the first reported electrochemical synthesis of inorganic films employing oxidation of hydroquinone. The layered potassium niobates can undergo cationic exchange to form (H3O)Nb3O8 and (H3O)4Nb6O17, and cationic exchange followed by heat treatment provided routes to two different forms of Nb2O5. In G


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(9) Yang, Y.; Jung, J. H.; Yun, B. K.; Zhang, F.; Pradel, K. C.; Guo, W.; Wang, Z. L. Flexible Pyroelectric Nanogenerators using a Composite Structure of Lead-Free KNbO3 Nanowires. Adv. Mater. 2012, 24, 5357−5362. (10) Choi, J.; Ryu, S. Y.; Balcerski, W.; Lee, T. K.; Hoffmann, M. R. Photocatalytic Production of Hydrogen on Ni/NiO/KNbO3/CdS Nanocomposites Using Visible Light. J. Mater. Chem. 2008, 18, 2371− 2378. (11) Jiang, L.; Qiu, Y.; Yi, Z. Potassium Niobate Nanostructures: Controllable Morphology, Growth Mechanism, and Photocatalytic Ability. J. Mater. Chem. A 2013, 1, 2878−2885. (12) Staedler, D.; Magouroux, T.; Hadji, R.; Joulaud, C.; Extermann, J.; Schwung, S.; Passemard, S.; Kasparian, C.; Clarke, G.; Gerrmann, M.; Le Dantec, R.; Mugnier, Y.; Rytz, D.; Ciepielewski, D.; Galez, C.; Gerber-Lemaire, S.; Juillerat-Jeanneret, L.; Bonacina, L.; Wolf, J.-P. Harmonic Nanocrystals for Biolabeling: A Survey of Optical Properties and Biocompatibility. ACS Nano 2012, 6, 2542−2549. (13) Cai, B.; Zhao, M.; Wang, Y.; Zhou, Y.; Cai, H.; Ye, Z.; Huang, J. A Perovskite-type KNbO3 Nanoneedles Based Biosensor for Direct Electrochemistry of Hydrogen Peroxide. Ceram. Int. 2014, 40, 8111− 8116. (14) Gasperin, M.; Lebihan, M. T. Hydration Mechanism of Lamellar Alkaline Niobates with the Formula A4Nb4O17 (A = K, Rb, Cs). J. Solid State Chem. 1982, 43, 346−353. (15) Nedjar, R.; Borel, N. M.; Raveau, B. H3ONb3O8 and HNb3O8 − New Protonic Oxides with a Layer Structure Involving Ion-Exchange Properties. Mater. Res. Bull. 1985, 20, 1291−1296. (16) Takagaki, A.; Lu, D.; Kondo, J. N.; Hara, M.; Hayashi, S.; Domen, K. Exfoliated HNb3O8 Nanosheets as a Strong Protonic Solid Acid. Chem. Mater. 2005, 17, 2487−2489. (17) Abe, R.; Shinmei, K.; Koumura, N.; Hara, K.; Ohtani, B. VisibleLight-Induced water Splitting Based on Two-step Photoexcitation between Dye-sensitized Layered Niobate and Tungsten Oxide Photocatalysts in the Presence of a Triiodide/Iodide Shuttle Redox Mediator. J. Am. Chem. Soc. 2013, 135, 16872−16884. (18) Dias, A. S.; Lima, S.; Carriazo, D.; Rives, V.; Pillinger, M.; Valente, A. A. Exfoliated Titanate, Niobate and Titanoniobate Nanosheets as Solid Acid Catalysts for the Liquid-Phase Dehydration of D-xylose into Furfural. J. Catal. 2006, 244, 230−237. (19) Ohnishi, R.; Katayama, M.; Takanabe, K.; Kubota, J.; Domen, K. Niobium-Based Catalysts Prepared by Reactive Radio-Frequency Magnetron Sputtering and Arc Plasma Methods as Non-Noble Metal Cathode Catalysts for Polymer Electrolyte Fuel Cells. Electrochim. Acta 2010, 55, 5393−5400. (20) Augustyn, V.; Come, J.; Lowe, M. A.; Kim, J. W.; Taberna, P.-L.; Tolbert, S. H.; Abruña, H. D.; Simon, P.; Dunn, B. High-rate Electrochemical Energy Storage through Li+ Intercalation Pseudocapacitance. Nat. Mater. 2013, 12, 518−522. (21) Zhao, Y.; Zhou, X.; Ye, L.; Tsang, S. C. E. Nanostructured Nb2O5 Catalysts. Nano Rev. 2012, 3, 17631. (22) Hiyoshi, M.; Lee, B.; Lu, D.; Hara, M.; Kondo, J. N.; Domen, K. Supermicroporous Niobium Oxide as an Acid Catalyst. Catal. Lett. 2004, 98, 181−186. (23) Atlas of Electrochemical Equilibria in Aqueous Solutions, 2nd ed.; Pourbaix, M., Ed.; NACE International: Houston, TX, 1974. (24) McDonald, K. J.; Choi, K.-S. A New Electrochemical Synthesis Route for a BiOI Electrode and Its Conversion to a Highly Efficient Porous BiVO4 Photoanode for Solar Water Oxidation. Energy Environ. Sci. 2012, 5, 8553−8557. (25) CRC Handbook of Chemistry and Physics, 92nd ed.; Lide, D. R., Ed.; CRC: Boca Raton, FL, 2011; pp 2011−2012. (26) Nassau, K.; Shiever, J. W.; Bernstein, J. L. Crystal Growth and Properties of Mica-Like Potassium Niobates. J. Electrochem. Soc. 1969, 116, 348−353. (27) Schäfer, H.; Gruehn, R.; Schulte, F. Modifications of Niobium Pentoxide. Angew. Chem., Int. Ed. Engl. 1966, 5, 40−52. (28) Limmer, S. J.; Kulp, E. A.; Switzer, J. A. Epitaxial Electrodeposition of ZnO on Au(111) from Alkaline Solution: Exploiting Amphoterism in Zn(II). Langmuir 2006, 22, 10535−10539.

other words, the deposition methods reported in this study can be used to form KNb3O8, K4Nb6O17, KNbO3, (H3O)Nb3O8, (H3O)4Nb6O17, and Nb2O5 (R and T phases) as electrodes. The versatility of this method was further demonstrated by the synthesis of CuNb2O6 and AgNbO3 films. These films were prepared by co-deposition of KNb3O8 and Cu2O, followed by thermal and chemical treatments. UV−vis spectra of all films and also the photocurrent measurements of CuNb2O6 and AgNbO3 films were obtained. CuNb2O6 and AgNbO3 showed p- and n-type behaviors, respectively. Because synthesis of Nb compounds from aqueous solutions has been challenging to date, the new methods reported in this study will open new pathways to the synthesis of various Nb compounds as thinfilm-type electrodes.


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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.langmuir.5b01665. LSV recorded in each of the plating solutions used in this study by sweeping the potential from the open circuit potential to the negative direction to identify the onset potential of benzoquinone reduction (Figure S1), highmagnification SEM images (Figure S2), and side-view SEM images of Nb oxide films discussed in the main text (Figure S3) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DESC0008707.

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REFERENCES

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Electrochemical Synthesis of Binary and Ternary Niobium-Containing

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