Electrostatic self-assembly behaviour of exfoliated Sr₂Nb₃O₁₀⁻ nanosheets and cobalt porphyrins: exploration of non-noble electro-catalysts towards hydrazine hydrate oxidation

Abstract

In our research, a convenient exfoliation/restacking route was used to fabricate a sandwich-structured nanocomposite of Sr₂Nb₃O₁₀/CoTMPyP [5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrinato cobalt] via electrostatic interaction between colloidal dispersion and cobalt porphyrin aqueous solution. The final self-assembled products were characterized by XRD, FTIR, UV–Vis, SEM, TEM, AFM, and ICP. During the exfoliation procedure, the well-dispersed Sr₂Nb₃O₁₀⁻ colloidal dispersion was obtained with the zeta potential value of − 44.2 mV. Additionally, the existence of unilamellar Sr₂Nb₃O₁₀⁻ nanosheet was evidenced by AFM, and zeta potential values of the reassembly process with the addition of CoTMPyP aqueous solution into the colloidal dispersion were measured by a Zetasizer Nano apparatus. Above all, the final Sr₂Nb₃O₁₀/CoTMPyP hybrid film displayed excellent electro-catalytic activities towards hydrazine hydrate oxidation with peak potential at 0.158 V in pH 7.0 PBS indicated by CV measurements; moreover, a detection limit of 3.52 × 10⁻⁵ M was obtained in the concentration range of 5 × 10⁻⁵–9.9 × 10⁻⁴ M at a signal-to-noise ratio of 3.0.

Introduction

Hydrazine hydrate was not only widely applied as a strong reducing agent, pharmaceutical intermediates, antioxidants, etc., but also considered as a type of fuel cell with high energy density, except for methanol, formic acid fuel cell [1,2,3,4]. Comparing with the efficient oxidation of methanol and formic acid limited to noble metal electro-catalysts only, the possible electro-catalytic oxidation of hydrazine hydrate on non-noble metal electrodes became the advantage of direct hydrazine fuel cell. Therefore, much attention was focused on the exploration of efficient non-noble electro-catalysts towards hydrazine hydrate oxidation [5,6,7,8,9,10]. It is worth mentioning that a well-known class of naturally occurring compounds in biological systems, porphyrins and their derivatives played an important role in the development of electro-catalysts for different reactions because of the intriguing redox properties [11]; in particular, transition metal porphyrins complexes were reported to exhibit excellent catalytic activity towards hydrazine hydrate oxidation [8]. To overcome the own limitations, or even to improve their properties, metalloporphyrin complexes have been usually immobilized into the rigid inorganic matrix [12, 13]. However, the tedious and time-resuming work-up was required for introducing functional guest spices in large volume into the interlayer by virtue of the traditional ion-exchange process; consequently, the convenient and effective route for fabricating functional hybrids was urgent to be developed.

On the other hand, two-dimensional layered metal oxide semiconductors (LMOSs) nanosheets derived from the parent layered materials have gained popularity in fundamental research [14,15,16,17,18] since the production of graphene with amazing functionalities. Consequently, an interesting osmotic swelling/exfoliation route was advanced for producing unilamellar LMOSs nanosheet in high quality through the permeation of bulky tetraalkylammonium cations (TAA⁺) and large amounts of H₂O molecules into the interlayer region to greatly weaken the interactions between neighbouring sheets [19, 20]. Owing to the structural diversity and extraordinary electronic properties, the obtained nanosheets with atomic or molecular thickness and micrometer-order lateral sizes were thus exploited for application in electronic, magnetic, optical, electrochemical, and catalytic nanodevices [17]; more interestingly, the negatively charged nanosheets can serve as promising building blocks for the construction of functional hybrid materials [21]. At present, the hybridization of a broad spectrum of guest species including transition metal complex [22, 23], transition metal nanoparticles [24, 25], and LDHs nanosheets [26, 27] with exfoliated LMOSs nanosheets have been achieved through electrostatic self-assembly reaction; nevertheless, the introduction of bulky organic functional pigments into the interlayer of LMOSs using the exfoliation/restacking method remained less studied [28].

As we all know, Dion–Jacobson (D–J)-type layered perovskites were highly favoured among two-dimensional LMOSs, one representative example of which was KSr₂Nb₃O₁₀ characteristic of triple-octahedral arrangements along the host layer (schematic illustration of the structure is in Fig. 1) [29], and the protonated product can be also dispersed in the TAAOH aqueous solution to form unilamellar nanosheet maintaining excellent properties of semiconductor from the parent [30]; herein, the promising inorganic matrix of KSr₂Nb₃O₁₀ was selected as the starting material, and Sr₂Nb₃O₁₀⁻ nanosheets were obtained by dispersing the protonated product into TBAOH [(C₄H₉)₄NOH] aqueous solution; what is more, the final Sr₂Nb₃O₁₀/CoTMPyP nanocomposites were fabricated via electrostatic interaction between Sr₂Nb₃O₁₀⁻ nanosheets colloidal dispersion and cobalt porphyrin aqueous solution (Fig. 2), and electro-catalytic measurements towards hydrazine hydrate oxidation adopting the Sr₂Nb₃O₁₀/CoTMPyP assembled hybrid film were taken.

Figure 1

Schematic illustration of KSr₂Nb₃O₁₀ structure

Figure 2

Schematic illustration of the reassembly process between Sr₂Nb₃O₁₀⁻ nanosheets and cobalt porphyrins

Experimental

Preparation of Sr₂Nb₃O₁₀/CoTMPyP nanocomposite

The host material of KSr₂Nb₃O₁₀ was synthesized through a conventional solid-state reaction referring to a previously reported literature [31]. The obtained product (1 g) was then converted into the protonic form of HSr₂Nb₃O₁₀ in 6 M HNO₃ (30 ml) for three times (24 h/per time). Concerning the preparation of exfoliated Sr₂Nb₃O₁₀⁻ nanosheets, it was realized by dispersing the acidified product into stoichiometric amounts of 10 (wt)% TBAOH aqueous solution with uniform agitation for 5 days (the molar ratio of TBA⁺/H⁺ = 1 was adopted), and the final translucent colloidal suspension was centrifuged at 3000 rpm for 10 min to remove the unexfoliated particles. In order to fabricate the Sr₂Nb₃O₁₀/CoTMPyP nanocomposite, 1 mM CoTMPyP aqueous solution (2, 3, 4 ml) was separately added into the obtained Sr₂Nb₃O₁₀⁻ nanosheets colloidal dispersion above (4, 3, 2 ml), and then the flocculated precipitates were washed with distilled water and anhydrous ethanol repeatedly and dried at 50 °C in a vacuum oven for 24 h. The final nanocomposites were labelled as Sr₂Nb₃O₁₀/CoTMPyP_0.5, Sr₂Nb₃O₁₀/CoTMPyP₁, Sr₂Nb₃O₁₀/CoTMPyP₂, according to the corresponding volume ratio of CoTMPyP aqueous solution and the colloidal dispersion (CoTMPyP/Sr₂Nb₃O₁₀⁻ = 0.5, 1, 2).

Characterization

Shimadzu FTIR-8400S spectrometer was employed for collecting infrared spectra of the products with the use of KBr pellets. UV–Vis absorption spectra were recorded with a UV–Vis spectrometer (UV-2550). A RINT 2000 diffractometer (Cu Kα radiation, λ = 0.154 nm, Rigaku) was used to obtain X-ray diffraction patterns of the samples with 2θ going from 2° to 40°. The scanning electron microscopic images of the samples were acquired by a JSM-5600 apparatus (JEOL), atomic force microscope (AFM) images were taken with a Bruker dimension edge SPM apparatus adopting the peak force tapping mode, and high-resolution transmission electron microscope (HRTEM) images were taken with the apparatus (JEOL, JEM-2010); the specimens were prepared by sonicating the products in ethanol for 10 min in advance and evaporating a drop of the resulting suspension onto a copper grid. Zeta potential was measured by a Malvern Zetasizer Nano instrument, and water at 25 °C was selected as the dispersion solvent. The concentration of Co element in the final hybrid solution was analysed by an inductively coupled plasma (ICP) spectrometer (CAP6300).

Electrochemical measurements

Electrochemical measurements were performed in a conventional three-electrode cell using a CHI660c electrochemical workstation, platinum wire electrode and a saturated calomel electrode (SCE) were used as the auxiliary electrode and the reference electrode, respectively, and the Sr₂Nb₃O₁₀/CoTMPyP₁ hybrid thin-film modified glass carbon electrode (GCE) was used as the working electrode by coating 7.5 μl 1 mg/ml Sr₂Nb₃O₁₀/CoTMPyP₁ aqueous dispersion on GCE and dried naturally for 24 h. With the purpose of investigating electro-catalytic properties of the final hybrid film towards hydrazine hydrate oxidation, the measuring techniques of cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were adopted in this experiment. All buffer solutions including Britton–Robinson (B–R) buffer solution and phosphate buffer solution (PBS) in the electrochemical measurements were purged with nitrogen for 20 min prior to the experiments.

Results and discussion

XRD analysis

The sintered product of KSr₂Nb₃O₁₀ was identified by X-ray diffraction analysis in Fig. 3, the basal spacing (d₀₀₂) of the starting material was calculated as 1.46 nm on the basis of the angle (6.03°) of (002) diffraction peak, and the formation of H₃O⁺ in the interlayer led the basal spacing of the acidified product an increase by 1.62 nm; upon the reassembly of Sr₂Nb₃O₁₀⁻ nanosheets and CoTMPyP cations, the (002) diffraction peaks of the three obtained nanocomposites shifted towards almost the same lower 2θ angle of ~ 4.3°, indicating that the basal spacings of Sr₂Nb₃O₁₀/CoTMPyP hybrids increased to ~ 2.05 nm, correspondingly. As observed in Fig. 3c, the XRD pattern of Sr₂Nb₃O₁₀⁻ nanosheets colloidal suspension, the disappearance of sharp peaks suggested the collapse of layered structure [32, 33]. On the other hand, the arrangement model of the Sr₂Nb₃O₁₀/CoTMPyP₁ hybrid can be postulated by the inclined angle of CoTMPyP molecules in the galley. According to the interlayer distance of the Sr₂Nb₃O₁₀/CoTMPyP₁ hybrid (the value was calculated as 0.59 nm by subtracting the Sr₂Nb₃O₁₀⁻ layer thickness of 1.44 nm) [16] and the CoTMPyP molecular dimension (18 × 18 Å, estimated by MM2 calculation), it was concluded that CoTMPyP molecules were located in the galley of the Sr₂Nb₃O₁₀/CoTMPyP₁ hybrid by an inclined angle of 19° to form a monolayer in Fig. 2.

Figure 3

XRD patterns of (a) KSr₂Nb₃O₁₀, (b) HSr₂Nb₃O₁₀, (c) Sr₂Nb₃O₁₀⁻ and (d) Sr₂Nb₃O₁₀/CoTMPyP_0.5, (e) Sr₂Nb₃O₁₀/CoTMPyP₁, (f) Sr₂Nb₃O₁₀/CoTMPyP₂

Zeta potential analysis

As can be seen from Fig. 4a, it presented the photograph of the reassembly process between Sr₂Nb₃O₁₀⁻ nanosheets colloidal suspension and CoTMPyP aqueous solution, the dark red flocculent precipitates appear at once as soon as CoTMPyP aqueous solution was added into Sr₂Nb₃O₁₀⁻ nanosheets colloidal dispersion, and the generation of the flocculent precipitates (i.e. the Sr₂Nb₃O₁₀/CoTMPyP nanocomposite) can be illustrated as the electrostatic interaction between Sr₂Nb₃O₁₀⁻ nanosheets with negative charge and cobalt porphyrin cations. Furthermore, the reassembly process can be finished within 20 min, and this is the advantage of exfoliation/restacking route over the traditional ion-exchange method.

Figure 4

a Photographs of Sr₂Nb₃O₁₀⁻ nanosheets colloidal suspension (left), CoTMPyP aqueous solution (middle), and the mixture of Sr₂Nb₃O₁₀⁻ nanosheets colloidal suspension and CoTMPyP aqueous solution (right). b Tyndall phenomenon of Sr₂Nb₃O₁₀⁻ nanosheets colloidal dispersion

Tyndall light scattering phenomenon of the Sr₂Nb₃O₁₀⁻ colloidal dispersion is exhibited in Fig. 4b, suggesting the occurrence of exfoliation to a certain degree [34]; in order to further investigate the delamination/restacking process, the measurement of the surface potential was made by a Zetasizer Nano instrument. The zeta potential value of Sr₂Nb₃O₁₀⁻ nanosheets colloidal dispersion was − 44.2 mV in the light of the zeta potential report in Figure S3; regarding the formation of Sr₂Nb₃O₁₀⁻ nanosheets, it can be explained as the penetration of TBA⁺ ions in large volume into the interlayer of the acidified product, and expansion of the interlayer spacing to greatly weaken the interactions of neighbouring nanosheets [20]. Additionally, the influence of the volume ratio between CoTMPyP aqueous solution and Sr₂Nb₃O₁₀⁻ nanosheets colloidal suspension on the zeta potential values was investigated; it can be observed that as the increase in CoTMPyP aqueous solution volume added into Sr₂Nb₃O₁₀⁻ nanosheets colloidal dispersion, the zeta potential values increased gradually (Fig. 5) and the pH values decreased from 12.21 to 6.55. The zeta potential absolute value was approximately zero at the volume ratio of 0.22, and the zeta potential value change in the reassembly process also provides the support for the occurrence of flocculation phenomenon between Sr₂Nb₃O₁₀⁻ nanosheets colloidal dispersion and CoTMPyP aqueous solution.

Figure 5

Relationship curve between zeta potential values and CoTMPyP/Sr₂Nb₃O₁₀⁻ volume ratios

IR spectra analysis

The successful introduction of CoTMPyP molecules into the galley of Sr₂Nb₃O₁₀⁻ laminate can also be identified by IR spectra analysis (Figure S1). The absorption peaks of KSr₂Nb₃O₁₀ host material were mainly in the range of 1000–400 cm⁻¹ (Nb–O stretching vibration); furthermore, the IR spectra of CoTMPyP molecules exhibited typical characteristic peaks such as 1640 cm⁻¹, and 1560, 1459, 1401 cm⁻¹, which were assigned to C=N stretching vibration in the pyridine substituent, and C=N, C=C stretching vibration in the porphyrin rings, respectively. Similarly, several absorption bands (1638, 1400, 1121, 924, and 582 cm⁻¹) were observed in the IR spectra of the Sr₂Nb₃O₁₀/CoTMPyP₁ nanocomposite, illustrating that the final hybrid contains not only the Sr₂Nb₃O₁₀⁻ host laminate, but also CoTMPyP molecules. Furthermore, the interaction between guest molecules and host materials caused a slight shift in the characteristic peaks for Sr₂Nb₃O₁₀/CoTMPyP compared with IR spectrum of pure CoTMPyP [35].

UV–visible absorption spectra analysis

As shown in Figure S2, typically, the Soret band and Q band of CoTMPyP aqueous solution appeared at 437 and 549 nm, respectively; however, the self-assembly reaction of Sr₂Nb₃O₁₀⁻ nanosheets and CoTMPyP cations led to red shifts in the Soret bands and Q bands of the obtained dark-brown hybrid films; in detail, Sr₂Nb₃O₁₀/CoTMPyP_0.5, Sr₂Nb₃O₁₀/CoTMPyP₁, Sr₂Nb₃O₁₀/CoTMPyP₂ hybrids exhibited 8-, 9-, 10-nm red shifts in the Soret bands, and 1-, 2-, 3-nm red shifts in Q bands, respectively, demonstrating the intense interaction between Sr₂Nb₃O₁₀⁻ laminate and CoTMPyP cations [36]. Additionally, owing to the different intermolecular interactions of stacked metalloporphyrin molecules, the occurrence of a broadening phenomenon was in the absorption spectra of the final reassembly products when compared to free CoTMPyP absorption bands, which was coincident with several previous reports [13, 37, 38].

Morphology analysis

The surface morphology of KSr₂Nb₃O₁₀, HSr₂Nb₃O₁₀, and the reassembly product, Sr₂Nb₃O₁₀/CoTMPyP₁, was acquired by SEM and TEM as shown in Fig. 6, comparing with that of the origin material, KSr₂Nb₃O₁₀, the smooth surface of the acidified product indicated higher crystallinity corresponding to XRD data of the acidified product, and the particle size of the protonated product ranges from 0.1 to ca. 10 μm. In order to confirm the existence of Sr₂Nb₃O₁₀⁻ nanosheets in the delamination process, the obtained colloidal dispersion was characterized by AFM adopting a peak force tapping mode. As can be seen clearly from Fig. 7a, there existed several nanosheets in irregular shape dispersing on the mica substrate, which provides strong evidence for the occurrence of exfoliation. On the side, the appearance of few stacked nanosheets reflected the randomness in the delamination process as well. On the basis of the height profile along the marked white line, the thickness of Sr₂Nb₃O₁₀⁻ nanosheet was measured as ca. 2.18 nm, and the additional thickness over the crystallographic thickness of the Sr₂Nb₃O₁₀⁻ laminate (1.44 nm) should be attributed to water molecules and counterions (TBA⁺) absorbed on the surface of the nanosheet [39,40,41], suggesting the production of unilamellar Sr₂Nb₃O₁₀⁻ nanosheet.

Figure 6

SEM images of a KSr₂Nb₃O₁₀, b HSr₂Nb₃O₁₀ and c Sr₂Nb₃O₁₀/CoTMPyP₁, TEM image of d Sr₂Nb₃O₁₀/CoTMPyP₁ and e the partial enlarged drawing of the rectangle in d

Figure 7

Typical AFM images of a Sr₂Nb₃O₁₀⁻ nanosheets, b Sr₂Nb₃O₁₀/CoTMPyP₁ hybrid deposited on a mica substrate and c, d the corresponding section height profile along the marked white line

After the self-assembly of Sr₂Nb₃O₁₀⁻ nanosheets and CoTMPyP molecules, the overall crystallinity of the final hybrid has decreased; nevertheless, sandwich-structure Sr₂Nb₃O₁₀/CoTMPyP₁ nanocomposite was constructed via electrostatic interaction between Sr₂Nb₃O₁₀⁻ nanosheets and cobalt porphyrin cations (Fig. 6c, d). As labelled in the partial enlarged drawing (Fig. 6e), the thickness was approximately 4.08 nm with double basal spacings; namely the basal spacing of Sr₂Nb₃O₁₀/CoTMPyP₁ nanocomposite was ca. 2.04 nm, which corresponds well with XRD data (2.05 nm) [42,43,44,45]. Furthermore, the morphology of Sr₂Nb₃O₁₀/CoTMPyP₁ nanocomposite was also analysed by AFM shown in Fig. 7b, and exfoliated Sr₂Nb₃O₁₀⁻ nanosheets in the delamination process didn’t disturb the reconstruction of the lamellar structure of the final hybrid. The final hybrid consisted of several stacked nanoplates, and the basal spacing of Sr₂Nb₃O₁₀/CoTMPyP₁ was measured as ca. 2.0 and 2.26 nm according to the height profile along the marked white line, which was in good agreement with XRD data (2.05 nm). Besides, the quantitative analysis of the final hybrid was conducted using an ICP instrument, and 2.4 µmol Co element in per litre Sr₂Nb₃O₁₀/CoTMPyP₁ nanocomposite solution was detected.

Electrochemical characterization

Figure 8 gives CV curves of CoTMPyP aqueous solution, Sr₂Nb₃O₁₀/CoTMPyP₁ hybrid film, and KSr₂Nb₃O₁₀ modified GCE in pH 12.0 B–R buffer solution at the scan rate of 100 mV s⁻¹. Owing to the sensitivity of metalloporphyrin molecules to pH conditions, undefined oxidation/reduction peaks were present in CV curves of CoTMPyP aqueous solution; on the contrary, the assembled hybrid film displayed two pairs of similar well-defined oxidation/reduction peaks, with the redox potentials of one couple of the oxidation/reduction peaks (I_a1, I_c1) at − 0.709 and − 0.754 V, and the redox potentials of the other couple of oxidation/reduction peaks (I_a2, I_c2) at − 0.853 and − 0.959 V, corresponding to Co^III/IITMPyP and Co^II/ITMPyP redox couple, respectively [46]. The reversible redox process can be illustrated by the following expressions: Co^IITMPyP⁴⁺ + e⁻ → Co^ITMPyP³⁺ (peak I_c2), Co^ITMPyP³⁺ − e⁻ → Co^IITMPyP⁴⁺ (peak I_a2), Co^IIITMPyP⁵⁺ + e⁻ → Co^IITMPyP⁴⁺ (peak I_c1), Co^IITMPyP⁴⁺ − e⁻ → Co^IIITMPyP⁵⁺ (peak I_a1). The presence of well-defined oxidation/reduction peaks and the increase in the peak current intensity in the final hybrid film revealed that the immobilization of CoTMPyP molecules on the support matrix of Sr₂Nb₃O₁₀⁻ was an effective method for utilizing excellent electrochemical performance of metalloporphyrin in harsh conditions.

Figure 8

CV curves of (a) KSr₂Nb₃O₁₀ modified GCE (dash line), (b) CoTMPyP aqueous solution (dot line) and (c) Sr₂Nb₃O₁₀/CoTMPyP₁ modified GCE in N₂-saturated pH 12.0 B–R buffer solution at 100 mV s⁻¹

Electro-catalytic properties of Sr₂Nb₃O₁₀/CoTMPyP hybrid film towards hydrazine hydrate oxidation

The electrochemical oxidation of N₂H₄·H₂O using Sr₂Nb₃O₁₀/CoTMPyP₁ modified GCE was attempted in 0.2 M pH 7.0 PBS, and a comparison of KSr₂Nb₃O₁₀/GCE, Sr₂Nb₃O₁₀/CoTMPyP₁ modified GCE and bare GC at 100 mV s⁻¹ was made (Fig. 9A). Concerning the electro-catalytic oxidation effect of bare GC and the host material modified GCE, they barely work in the N₂H₄·H₂O oxidation process, while the oxidation potential of Sr₂Nb₃O₁₀/CoTMPyP₁ modified GCE appeared at 0.158 V, suggesting that the obtained assembled hybrid film through the exfoliation/restacking route can promote electrochemical oxidation process towards N₂H₄·H₂O effectively.

Figure 9

A CV curves of (a) bare GCE (dash line), (b) KSr₂Nb₃O₁₀ modified GCE (dash dot line), and (c) Sr₂Nb₃O₁₀/CoTMPyP₁ modified GCE (solid line) in N₂-saturated pH 7.0 PBS containing 3.92 mM N₂H₄·H₂O at 100 mV s⁻¹. B CV curves of Sr₂Nb₃O₁₀/CoTMPyP₁ modified GCE in N₂-saturated pH 7.0 PBS containing 3.92 mM N₂H₄·H₂O at 10, 20, 30, 40, 50, 60, 70, 100, 150, 200, 300, and 400 mV s⁻¹; the inset is the relationship curve between I_pa and v^1/2

Additionally, the influences of scan rate on CV behaviour of the final hybrid film were investigated in 0.2 M pH 7.0 PBS shown in (Fig. 9B). With the increase in the scan rate from 10 to 400 mV s⁻¹, the oxidation peak current increases gradually, and the oxidation peak potential moved towards negative, indicating the irreversibility of hydrazine hydrate oxidation process on Sr₂Nb₃O₁₀/CoTMPyP₁ modified GCE. According to the linear equation of I_pa (μA) = 6.11 + 61.15 v^1/2 (V^1/2 s^−1/2) (R = 0.9990) of the relationship curve plotted in the inset (Fig. 9B), a conclusion can be drawn that peak current (I_pa) was proportional to square root of the scan rate (v^1/2), demonstrating that electrochemical oxidation process on the surface of Sr₂Nb₃O₁₀/CoTMPyP₁ modified GCE was controlled by hydrazine hydrate diffusion. In addition, the inverted shape peaks were found in the CV curves of Sr₂Nb₃O₁₀/CoTMPyP towards hydrazine hydrate oxidation analogous to previous literatures [47, 48], which was probably ascribable to the formation of Co^IIITMPyP–N₂H₄ complex in the electrochemical oxidation process, and thus, the possible mechanism of N₂H₄·H₂O oxidation that occurred on the modified electrode was proposed as follows referring to related literatures [49, 50]:

$$ {\text{PCo}}^{\text{III}} + {\text{N}}_{2} {\text{H}}_{4} \to {\text{PCo}}^{\text{III}} \cdots {\text{N}}_{2} {\text{H}}_{4} $$

(1)

$$ {\text{PCo}}^{\text{III}} \cdots {\text{N}}_{2} {\text{H}}_{4} + 4{\text{H}}_{2} {\text{O}} \to {\text{PCo}}^{\text{II}} + {\text{N}}_{2} + 4{\text{H}}_{3} {\text{O}}^{ + } + 3{\text{e}}^{ - } $$

(2)

$$ {\text{PCo}}^{\text{II}} - {\text{e}}^{ - } \to {\text{PCo}}^{\text{III}} $$

(3)

P in the formula represents porphyrin ring, and the rate of the whole process is determined by the formation of Co^IIITMPyP–N₂H₄ complex, namely the second step.

At last, the electrochemical determination of N₂H₄·H₂O using Sr₂Nb₃O₁₀/CoTMPyP₁ modified GCE was attempted with the peak current (I_pa) and hydrazine hydrate concentration (c) in hydrazine hydrate concentration of 0.05–0.99 mM in 0.2 M pH 7.0 PBS. As can be inferred from the relationship curve of the inset (Fig. 10), the peak current was proportional to hydrazine hydrate concentration with the calibration equation of I (μA) = 0.06 + 4.05c (mmol L⁻¹) (r = 0.9957); hence, a detection limit was calculated as 3.52 × 10⁻⁵ M at a signal-to-noise ratio of 3.0.

Figure 10

DPV curves of Sr₂Nb₃O₁₀/CoTMPyP₁ modified GCE in N₂-saturated pH 7.0 PBS solution with hydrazine hydrate concentration ranging from 0.05 to 0.99 mM; the inset is the relationship curve between I_pa and c

Conclusions

The successful introduction of functional metalloporphyrin molecules (CoTMPyP) in large volume into the interlayer of KSr₂Nb₃O₁₀ through the convenient exfoliation/restacking route for the first time, which was confirmed by XRD, FTIR, UV–Vis, SEM, TEM, AFM, and ICP. Comparing with the crystallographic thickness of the Sr₂Nb₃O₁₀⁻ laminate (1.44 nm), the observed thickness of Sr₂Nb₃O₁₀⁻ nanosheet (2.18 nm) was likely to be homogeneously unilamellar, and the basal spacing of Sr₂Nb₃O₁₀/CoTMPyP₁ nanocomposite was measured as 2.0, 2.26 nm (AFM) and 2.04 nm (TEM) corresponding well to XRD data (2.05 nm). Besides, the well-dispersed and stable Sr₂Nb₃O₁₀⁻ nanosheets colloidal dispersion with the zeta potential value of − 44.2 mV was obtained, and the zeta potential value was close to 0 mV when the volume ratio of CoTMPyP aqueous solution to Sr₂Nb₃O₁₀⁻ nanosheets colloidal dispersion was 0.22. The fabrication of Sr₂Nb₃O₁₀⁻ and CoTMPyP cations through electrostatic self-assembly will facilitate the development of a variety of novel functional nanocomposites based on D–J-type perovskite nanosheets. Eventually, the final self-assembled hybrid film prepared by drop-coating method exhibited excellent electro-catalytic activities towards N₂H₄·H₂O oxidation in pH 7.0 PBS, which makes it a promising electro-catalyst towards N₂H₄·H₂O oxidation; meanwhile, this paper also provides theoretical basis for oxidation of N₂H₄·H₂O by virtue of functional nanocomposites associated with metalloporphyrin complex. Finally, a detection limit was estimated as 3.52 × 10⁻⁵ M according to a signal-to-noise ratio of 3.0.