Introduction

Nanostructured layered metal oxides are a kind of amazing materials with unique two-dimensional structure. Structural, textural, and compositional modifications of layered metal oxides have been investigated to develop new materials with tailored properties, such as catalysis [1], photocatalysis [2–4], electrooxidation [5] and photoluminescent behaviors [6, 7], etc. K4Nb6O17 · 3H2O is a unique semiconductor material possessing two alternative interlayer spaces (interlayer I and II) formed by [Nb6O17]− layers, between which hydrated and non-hydrated K+ ions exist to maintain the charge balance [8, 9]. The hydrated K+ ions in interlayer I can be directly exchanged by both monovalent and multivalent ions [10, 11], as is shown in Fig. 1. Intercalation reactions of K4Nb6O17 with metal ions [12, 13], methylviologen [10, 14–17], [Ru(phen)3]2+ ions [18], [Ru(bpy)3]2+ [19–23], rhodamine 6G (R6G) dye [24], porphyrin [25, 26] have been achieved, the photocatalytic, photoelectrochemical, and photo-induced electron-transfer behaviors of the intercalation nanocompound have been extensively investigated. However, although the electrochemical behaviors of the intercalated ions have been studied, the prospective utilization of the nanocomposites as modified electrodes is not widely discussed.

Fig. 1
figure 1

The structural model of K4Nb6O17 · 3H2O showing two different interlayers

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Methylene blue (MB) is a kind of basic dye with plane structure (Fig. 2), which is widely used for electrochemical applications, such as catalyst-mediator in electrochemical biosensors. To overcome the water-soluble disadvantages, MB has been immobilized in various matrices such as silicate [27], barium phosphate [28], TiO2 [29], mordenite-type zeolite [30], zirconium phosphate [31], layered manganese oxide [32]. MB+ intercalated K4Nb6O17 has been synthesized by ion-exchange [33] and electrostatic self-assembly deposition (ESD) [34] method, but the electrochemical behavior of intercalated MB+ was not reported in detail.

Fig. 2
figure 2

The molecular structure (a) and structural model (b) of methylene blue cation

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In the present work, MB was intercalated into interlayer I of K4Nb6O17 by host–guest ion exchange method by use of layered MV2+–K4Nb6O17 as precursor. The MB+–K4Nb6O17 nanocomposite was characterized by X-ray diffraction, IR, TGA, elemental analysis, and UV spectroscopy. The electrochemical behaviors of the MB+–K4Nb6O17 hybrid thin film electrode were studied.

Experimental

Analytical Nb2O5 and KOH were purchased from Sinopharm Chemical Reagent Co., Ltd., methylviologen chloride and methylene blue were purchased from Tokyo Kasei, all reagents were used without further purification.

The layered compound, K4Nb6O17 · 3H2O, employed here was prepared by calcination of a 2.1:3.0 molar mixture of K2CO3 and Nb2O5 at 1100 °C for 10 h, according to the procedures reported by Nassau et al. [35]. Figure 1 shows the layered structure of K4Nb6O17 · 3H2O. K4Nb6O17 consists of octahedral units of NbO6, which form a two-dimensional layered structure via bridging oxygen atoms. The layers are negatively charged, and K+ ions exist between the layers to compensate for the negative charges of the layers.

The intercalation compound of K4Nb6O17 with MB+ is generally very difficult to prepare by a direct reaction because of the bulkiness of MB+. A two-step intercalation was thus attempted by adopting methylviologen-K4Nb6O17 intercalation compounds as the intermediate. The methylviologen cation (MV2+) can be smoothly intercalated into interlayer I of K4Nb6O17. [10, 14–17] The nano-structured hybrid MV2+–K4Nb6O17 was prepared by treating K4Nb6O17 · 3H2O with an aqueous solution of excess methylviologen chloride and then allowing it to stand for 3 weeks at 70 °C. The resultant product was washed with deionized water until the MV2+ absorption could not be detected at 257 nm in the filtrate solution [22]. To make the MB+–K4Nb6O17 hybrid film, 150 μL aqueous suspension of MV2+–K4Nb6O17 composite was cast onto a quartz glass plate (20 × 40 mm2) to obtain an optically transparent film, and then the plate was placed in a 5 mM aqueous solution of methylene blue for 2 weeks. Rinsed the plate with deionized water carefully and dried to obtain a dark blue MB+–K4Nb6O17 nanocomposite thin film. All the procedures were prepared at room temperature. A thin film of MB+–K4Nb6O17 nanocomposite was prepared on the surface of a glass carbon electrode (GCE) with the same procedure for electrochemical investigation.

XRD patterns of the MV2+–K4Nb6O17 and MB+–K4Nb6O17 hybrid were collected with a M21X (MAC Co., Ltd.) diffractometer with monochromatic Cu Kα radiation (λ = 0.15406 nm) with 2θ going from 1.5° to 40° in 1° steps. UV absorption spectra of the two hybrids were carried out using a UV–vis spectrometer (UV-2550). IR spectra were measured on a WGH-30/6 double-beam IR-spectrometer with the use of KBr pellets. Thermal gravimetric analysis (TGA) was carried out on a Shimadzu DTG-60 apparatus at a heating rate of 20 °C min−1 from room temperature to 750 °C in air. Elemental analysis (EA) was performed using a Perkin Elmer 2400-CHN elemental analyzer.

The electrochemical experiments were carried out in a conventional three-electrode electrochemical cell at room temperature, with a platinum electrode as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The MB+–K4Nb6O17 nanocomposite thin film modified GC electrode was used as the working electrode. The acting electrolyte was 0.1 mol L−1 HCl solution. The solution was bubbled with N2 for 20 min before examination in order to avoid the influence of oxygen. CV scans were carried out on a CHI600b electrochemical workstation at scan rate of 10–500 mV s−1.

Results and discussion

Characterization of MB+–K4Nb6O17 nanocomposite

K4Nb6O17 · 3H2O was identified by powder X-ray diffraction analysis (Fig. 3a). Sharp peaks indicated the high crystallinity of the K4Nb6O17 · 3H2O. The d020 value is regarded as basal spacing corresponding to the sum of the two adjacent interlayer spaces. The XRD pattern of hydrated potassium niobate K4Nb6O17 · 3H2O (Fig. 3a) exhibited a (020) weak diffraction peak at 1.88 nm and an intense peak (040) at 0.94 nm, indicating that water molecules are intercalated only into the interlayer I [18]. The basal spacing of MV2+–K4Nb6O17 (Fig. 3b, 2.26 nm) was larger than that of K4Nb6O17 · 3H2O, indicating the substituent of MV2+ cations for water molecular in interlayer I. The interlayer I expansion of MV2+–K4Nb6O17 was calculated as 1.03 nm by subtracting the sum of thickness of the interlayer II and niobate layer of K4Nb6O17 · 3H2O (1.23 nm) from the observed basal spacing (2.26 nm) [12].

Fig. 3
figure 3

The X-ray diffraction patterns of a K4Nb6O17 · 3H2O, b MV2+–K4Nb6O17 thin film, and c MB+–K4Nb6O17 thin film

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As was shown in Fig. 3c, the 2θ angle of the (020) diffraction peak of MB+–K4Nb6O17 was smaller than that of the mediate MV2+–K4Nb6O17, showing that the intercalation of MB+ enlarged the interlayer I spacing of K4Nb6O17. The interlayer I spacing of MB+–K4Nb6O17 was calculated as 1.71 nm, which is larger than the spacing of MV2+–K4Nb6O17 deposited from ESD precipitations [34]. Considering the rectangular dimensions of MB molecular (1.70 × 0.76 × 0.325 nm3) [36], we estimate that MB+ cations are placed in interlayer I in two ways, which is shown in Fig. 4. The MB+ cations may form a double layer parallel to the [Nb6O17]4− layers, or a single layer standing vertically to the [Nb6O17]4− layers.

Fig. 4
figure 4

The proposed intercalation model of MB+–K4Nb6O17: a double layer parallel to [Nb6O17]4− layers, b single layer vertical to [Nb6O17]4− layers

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The intercalation of MB into of K4Nb6O17 was confirmed by IR spectra in Fig. 5. Characteristic infrared absorption peaks of MB [27, 29, 37] in Fig. 5b at 1604 cm−1 and 1494 (stretching modes of the aromatic rings), 1400 cm−1 (C–N symmetric stretch), 1358 cm−1 (–CH3 symmetric deformation), and of K4Nb6O17 [38, 39] at 905 cm−1 and 539 cm−1 (Nb–O stretching vibration) in Fig. 4a appeared in the IR spectrum of MB+–K4Nb6O17 (Fig. 5c). This confirms that MB has been intercalated into the interlayer spaces of niobate successfully without disturbing the structure of MB. However, there were some differences between the IR spectra of MB+–K4Nb6O17 and the spectra of MB and K4Nb6O17. For MB, the adsorption band at 1604 cm−1 shifted to 1608 cm−1 in the IR adsorption of the nanocomposites, for K4Nb6O17, the adsorption band at 539 cm−1 shifted to 529 cm−1. It can be contributed to the bonding interaction between the Nb–O groups of the niobate and the nitrogen of the aromatic ring of MB [27, 38].

Fig. 5
figure 5

IR spectra of a K4Nb6O17 b MB, and c MB+–K4Nb6O17 nanocomposite

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On the base of the elemental analysis data (C = 10.17%, H = 1.38%, N = 2.25%), the molecular formula of the MB+–K4Nb6O17 nanocomposite is assigned as MB0.63–K3.37Nb6O17 · 2.55H2O. The C/N mole ratio of the CHN analysis is calculated as 5.27, which is in good agreement with the observed value, 5.33, proves that the methylviologen ions in the niobate interlayer were replaced by MB+. Figure 6 gives the TG curve of the MB+–K4Nb6O17 nanocomposite. We explained the thermal behavior of the MB+–K4Nb6O17 nanocomposite with a two-step weight loss process. The first weight loss step from room temperature to 330 °C is caused by the release of intercalated water, which is dependent on the ambient conditions because of the sensitivity of K4Nb6O17 to the atmospheric humidity [25, 26]. The second weight loss above 330 °C is due to the decomposition of the organic portion in the nanocomposite interlayer. The total weight loss (19%) is consistent with the sum of the water and MB+ content determined by the elemental analysis.

Fig. 6
figure 6

TGA curve of MB+–K4Nb6O17 nanocomposite

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The charge density of the [Nb6O17]4− layer is 0.126 nm2 per negative charge [16], so the projected area of each MB+ ion in interlayer I can be calculated as 0.126 × 2/0.63 = 0.4 nm2. Judging from the dimensions of MB molecular, it can be concluded that the intercalation models in Fig. 4 (double layer parallel/monolayer perpendicular) are reasonable.

UV–vis optical absorption of MB+–K4Nb6O17 nanocomposite is shown in Fig. 7b, the UV spectrum of MV2+–K4Nb6O17 mediate (Fig. 7a) is also given for comparison. There is a broad absorption band in the visible light region in curve b, presenting a maximum absorbance at 586 nm and a shoulder at 680 nm, which confirms the presence of MB+ in interlayer I of K4Nb6O17. It is known that MB+ has strong tendency to aggregate in aqueous solution, the typical maximum absorbance peaks locate at around 665 nm and 610 nm, ascribing to monomer and dimer, respectively [27, 40–44]. The shift of the MB monomer (665 nm) peak toward shorter wavelength clearly denotes that MB+ cations in K4Nb6O17 interlayer are aggregated. Considering the position of the maximum (586 nm) and the shape of the signal, we suggest that MB is mainly present as trimers and dimers, while the proportion of monomers is relatively low [36, 45]. This result is corresponding to the MB+–K4Nb6O17 nanocomposite obtained by ESD [34].

Fig. 7
figure 7

UV spectrum of MV2+–K4Nb6O17 (a) and MB+–K4Nb6O17 (b) nanocomposites

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Electrochemical behavior of MB+–K4Nb6O17 nanocomposite thin film

The CV curve of MB in aqueous solution at 50mV s−1 scan rate is shown in Fig. 8a. There are a couple of sensitive oxidation/reduction peaks with redox potentials at 0.205 V and 0.174 V, with the midpoint potential [Em = (Epa + Epc)/2] of 0.190 V. The peak separation [∆Ep = (Epa − Epc)] is 31 mV, which is between the ∆Ep value of individual electron transfer reaction (59 mV) and two-electron transfer reaction (28.5 mV), indicating a two-step individual electron transfer reactions of MB dimer [46, 47].

Fig. 8
figure 8

Cyclic voltammograms of a MB (0.05 mmol L−1) and b MB+–K4Nb6O17 thin film in 0.1 mol L−1HCl solution, scan rate: 50 mV s−1

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The cyclic voltammogram of MB+–K4Nb6O17 nanocomposite thin film on GCE is shown in Fig. 8b. The redox potentials at 0.202 V and 0.148 V with a midpoint potential of 0.175 V are similar with the peaks of MB in solution except the larger peak separation of 54 mV. There is a shift of Epc to more negative values and a shift of Epa to more positive values with increasing the scan rate (Fig. 9). We describe this redox peaks as a two-electron quasi-reversible redox process of MB dimer [31, 48–50]. The linear dependence of the cathodic peak current (Ic) on the square root of the scan rate (Fig. 10) displays a planar diffusion controlled reduction behavior of MB+ cations in niobate interlayer. The ∆Ep increased from 22 mV up to 143 mV when the scan rate varied from 10 to 500mV s−1, indicating a slow electron diffusion process of the MB+ cations in the interlayer space at high scan rates. This is due to the semiconducting nature of the niobate matrix [31].

Fig. 9
figure 9

CVs of MB+–K4Nb6O17 thin film at different scan rates (10, 20, 30, 40, 50, 60, 70, 80, 100, 150, 200, 250, 300, 400, 500 mV s−1 from inner to outer)

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Fig. 10
figure 10

Ic ~ υ 1/2 curve for MB+–K4Nb6O17 layer nanocomposite thin film in 0.1 mol L−1 HCl solution

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The ideal linear relationship in Fig. 10 indicates a fine mass transfer process, which is suitable for utilization as electrode modifying. In order to test the electrochemical stability of the MB+–K4Nb6O17 hybrid film, the modified GCE was tested again after exposure in air for 7 days. The cathodic and anodic currents of redox peaks I were still 90% of initial value at almost the same peak potential, confirming the good immobilization of MB in niobate interlayer spacings.

Conclusion

Laminar nanomaterial K4Nb6O17 was synthesized through solid-phase synthesis and characterized by XRD. Methylviologen was intercalated into interlayer I of K4Nb6O17 by ion exchange reaction. Then methylene blue (MB) substituted the MV2+ cations through guest–guest ion exchange method. The two hybrid nanocomposites were characterized using IR, UV, XRD. UV pattern of MB+/K4Nb6O17 thin film indicated that the intercalated MB+ existed mainly as dimer and trimer, while the portion of monomer is fairly low. The cyclic voltammogram of the MB+–K4Nb6O17 nanocomposite film exhibited a pair of distinct reductive and oxidative peaks, representing a two-electron redox process of MB dimer. The cathodic current exhibited a fine diffusion-controlled process, and the electrochemical stability of the hybrid film was also proved. We predict that novel MB+–K4Nb6O17 nanocomposite has possibility to be used as electrode modifying material.