III. RESULTS AND DISCUSSION

As displayed in Fig. 1(a), the characteristic peaks of CsTi₂NbO₇ can be clearly observed, which is in line with JCPDS Card (No. 73-0680),^{Reference Liu, Zhang, Wang, Xu, Chen, Wang, Xi and Hou44} indicating the highly crystalline nature and purity of CsTi₂NbO₇. After calcinating CsTi₂NbO₇ with melamine, the characteristic peaks of CsTi₂NbO₇ are still retained in the resulted NTCN, confirming the maintenance of the layered structure. It is worth noting that a new peak located at 6.16° (d = 1.25 nm) can be clearly seen in NTCN due to the intercalation of g-C₃N₄ into the interlayers of CsTi₂NbO₇ to form layer-by-layer stacking structure between g-C₃N₄ layers and CsTi₂NbO₇ layers.^{Reference Gunjakar, Kim, Kim, Kim and Hwang45} Besides, the characteristic peaks of g-C₃N₄ are not found in the XRD pattern of NTCN, which may be due to the overlapped peaks. Considering the thickness of the [Ti₂NbO₇]⁻ layer to be 0.58 nm,^{Reference Liu, Zhang, Wang, Xu, Chen, Wang, Xi and Hou44} the gallery height is calculated to be 0.67 nm resulting from the intercalation g-C₃N₄. As the thickness of the single-layer g-C₃N₄ nanosheet is 0.325 nm,^{Reference Xu, Zhang, Shi and Zhu46} it can be inferred that two-layered g-C₃N₄ nanosheets is in situ intercalated into the interlayers of CsTi₂NbO₇ owing to the space-confined effect, leading to the formation of the CsTi₂NbO₇/g-C₃N₄ heterojunction structure.

FIG. 1.

XRD patterns (a) and FT-IR spectra (b) of CsTi₂NbO₇, g-C₃N₄, and NTCN composite.

The local structure of the samples was investigated by FT-IR spectra shown in Fig. 1(b). In the spectrum of pure g-C₃N₄, the bands at 1637, 1460, 1403, 1323, and 1241 cm⁻¹ are assigned to the typical stretching modes of C–N heterocycles.^{Reference Liu, Wu, Ji, Zhu, Hou, Yang, Jiang, Wang, Tian, Chen and Hou47} Besides, the representative band at 805 cm⁻¹ of g-C₃N₄ is attributed to the characteristic breathing mode of s-triazine.^{Reference Xia, Ji, Di, Wang, Yin, Zhang, He and Li48} For CsTi₂NbO₇, the bands at the range of 950–550 cm⁻¹ are characteristic Ti–O and Nb–O stretching vibrations.^{Reference Park, Shin, Song, Park and Paik49} After combing g-C₃N₄ with CsTi₂NbO₇, a significant shift of the characteristic bands can be observed in the resulted NTCN composite in comparison with g-C₃N₄ and CsTi₂NbO₇, owing to the presence of a strong interaction between these two compounds, resulting from the formation of the CsTi₂NbO₇/g-C₃N₄ heterojunction structure.^{Reference Zhang, Huang, Zhang, Yuan and Dong50}

The morphology of samples is examined by SEM and TEM. It can be clearly observed that CsTi₂NbO₇ possesses smooth surface and displays a typical nanobelt structure with a width size of 1–2 μm [Fig. 2(a)]. After heating CsTi₂NbO₇ with melamine, the nanobelt structure is still maintained in the resulted NTCN composite shown in Fig. 2(b). The unique long nanobelt structure can act as a channel for transport of charge carriers, which is beneficial for facilitating the separation of electron–hole pairs, and then transferred to different reaction sites for further reactions.^{Reference Liang, Guo, Cao, Tian and Lui42} Besides, CsTi₂NbO₇ can be doped by the nitrogen atoms to form N–CsTi₂NbO₇ during this process, due to the ammonia-based gas released from pyrolysis of melamine. From the TEM image [Fig. 2(c)], a core–shell nanobelt structure can be clearly seen in the NTCN composite, in which g-C₃N₄ is densely assembled on the surface of N–CsTi₂NbO₇. As shown in Fig. S2, the HRTEM image of NTCN confirmed the formation of heterojunction between g-C₃N₄ and CsTi₂NbO₇. The interplanar spacing of lattice fringes is about 0.32 nm, in line with the (031) plane of CsTi₂NbO₇. Combined with XRD results, the formed g-C₃N₄ is located on both the interlayers and surface of CsTi₂NbO₇ to form CsTi₂NbO₇/g-C₃N₄ heterojunction, which is favor of the separation of electron–hole pairs.^{Reference Liu, Sun, Wu, Liang, Huang, Chen and Hou51}

FIG. 2.

SEM photograph of (a) CsTi₂NbO₇, (b) NTCN composite, and (c) TEM image of the NTCN composite.

The morphology and distribution of elements in NTCN composite are further investigated through high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and elements mapping. In Fig. 3(a), the nanobelt with layered structure can be clearly observed. For a deep insight of the nanostructure, the element mappings are measured. In the merged image [Fig. 3(b)], the different colors of C, N, O, Ti, Nb, and Cs regions are highly coincident, indicating that g-C₃N₄ is successfully assembled with CsTi₂NbO₇. However, C, N, O, Ti, Nb, and Cs elements are uniformly dispersed in the NTCN composite.

FIG. 3.

Element map of the NTCN composite by HAADF–STEM. (a) HAADF–STEM image of the NTCN composite. (b) Combined C/N/O/Ti/Nb/Cs chemical maps of the NTCN composite. The distribution of (c) C, (d) N, (e) O, (f) Ti, (g) Nb, and (h) Cs in the NTCN composite.

Figure 4 shows the electrochemical impedance spectroscopy (EIS) of CsTi₂NbO₇, g-C₃N_4, and NTCN electrodes. It shows that the diameter of the Nyquist circle of the NTCN/GCE is smaller than CsTi₂NbO₇ and g-C₃N₄ in the high frequency region, indicating a promoting charge transfer behavior at the interface of electrode–electrolyte.^{Reference Hou, Zuo, Dagg and Feng52} The reasons might be due to the fact that the construction of CsTi₂NbO₇/g-C₃N₄ heterojunction can promote the separation of electron–hole pairs and reducing the recombination rate of electro-induction electrons and holes.^{Reference Xiang, Yu and Jaroniec53}

FIG. 4.

EIS of CsTi₂NbO₇, g-C₃N₄ and NTCN in 0.5 mmol/L K₃Fe(CN)₆ containing 0.1 mol/L KCl at open potential.

The electrochemical performances of the samples are also investigated by CVs [Fig. 5(a)], which were measured in 5.0 mmol/L K₃Fe(CN)₆ containing 0.1 mol/L KCl solution at 50 mV/s. A pair of quasi-reversible redox peaks can be clearly seen in NTCN/GCE with a higher ampere response, demonstrating an increased electroactive surface area and effective acceleration of electron transfer rate in NTCN/GCE. Furthermore, the corresponding peak-to-peak separation value in NTCN/GCE (ΔE _p = 0.117 V) is smaller than that of CsTi₂NbO₇/GCE (ΔE _p = 0.224 V) and g-C₃N₄/GCE (ΔE _p = 0.129 V). It is supposed that the reasons are due to that (i) the inserted g-C₃N₄ has broaden the distance of layered CsTi₂NbO₇, leading to larger surface areas and more active sites of the NTCN composite so as to enhance the diffusion rate of nitrite molecules on the electrode. The nitrogen adsorption–desorption isotherms for the samples are shown in Fig. S3. The BET surface area of NTCN is larger than those of CsTi₂NbO₇ and g-C₃N₄. Thus, the larger surface areas contribute to the higher electrocatalytic activity. (ii) The construction of the heterojunction structure will speed up the separation and efficient transfer of electrogenerated charge carriers.

FIG. 5.

(a) CVs of CsTi₂NbO₇/GCE, g-C₃N₄/GCE, and NTCN/GCE recorded in 5.0 mmol/L K₃Fe(CN)₆ containing 0.1 mol/L KCl solution at 50 mV/s (b) CVs of NTCN/GCE in 0.1 mol/L phosphate buffer solution (PBS; pH = 7.0) containing 3.28 mmol/L nitrite at different scan rates. (c) The plots of oxidation peak current to the square root of scan rates. (d) The linear relationship of E _pa versus log v.

For NTCN/GCE, the influence of scan rates on the oxidation of nitrite (3.28 mmol/L) is also evaluated by CVs [Fig. 5(b)]. It is found that the oxidation peak current increases gradually with the increase of scan rates. An excellent linearity between peak current and square root of scan rates (v) in 40–140 mV/s can be found in Fig. 5(c) with the linear relationship of I _pa (μA) = 275v ^1/2 (V/s) + 27.0 (n = 6, R = 0.9995), indicating a classical diffusion-controlled electron transfer process.^{Reference Pan, Xu, Zhang, Li, Wang, Ma, Liu, Zhang and Tong16} Here n and R represent the linear equation from six groups of data and linear correlation coefficient, respectively. Additionally, the anodic potentials of nitrite shift slightly along with the increase of scan rates, and the regression equation can be expressed as E _pa (V) = 0.153 log v + 0.664 (n = 6, R = 0.9993) [Fig. 5(d)], which suggests a kinetic limitation in the reaction between the NTCN/GCE and nitrite. For an irreversible process, the number of electron transfer can be calculated according to the Laviron’s equation^{Reference Laviron54,Reference Afkhami, Soltani-Felehgari, Madrakian and Ghaedi55} :

(1)$${E_{{\rm{pa}}}} = {{2.303RT} \over {2(1 - \alpha )n\alpha F}}\log v + K\quad ,$$

(2)$${E_{{{\rm{p}} \over {\rm{2}}}}} - {E_{{\rm{pa}}}} = {{1.857RT} \over {\alpha F}}\quad ,$$

where α is the electron transfer cofficient, which can be obtained from Eq. (2). E _p/2 is the potential when the current is the half of the peak value. n is the number of electron transferred in the rate-determining step. R, T, and F correspond to the gas constant, temperature, and Faraday constant, respectively. From the slope of E _pa − log v, the n value is estimated to be 0.83. It means that the rate-determining step in the reaction is a single electron transfer process. This indicates that there are two successive reactions of nitrite oxidation on the modified electrode with the oxidation of NO₂⁻ to NO₂ as well as the rapid disproportionation of NO₂ into NO₂⁻ and NO₃⁻, respectively [Eqs. (3) and (4)].^{Reference Pham, Li, Han, Huynh-Nguyen, Le, Ko, Kim and Seong56}

(3)$${\rm{NO}}_2^ - - {{\rm{e}}^ - } \to {\rm{N}}{{\rm{O}}_2}\quad ,$$

(4)$$2{\rm{N}}{{\rm{O}}_2} + {{\rm{H}}_2}{\rm{O}} \to {\rm{NO}}_3^ - + {\rm{NO}}_2^ - + 2{{\rm{H}}^ + }\quad .$$

A. Mechanism of the electrocatalytic enhancement

The UV-visible diffuse reflectance spectra are investigated for further understanding the electrocatalytic enhancement of the NTCN nanocomposite (Fig. 6). The absorption edges of CsTi₂NbO₇, CsTi₂NbO₇/g-C₃N₄ heterojunction, and g-C₃N₄ are around 330, 414, and 445 nm, corresponding to the band-gap value of 3.75, 2.99, and 2.78 eV, respectively. For CsTi₂NbO₇, there is an only absorption band in the UV region owing to the band-to-band transition.^{Reference Liu, Liang, Han, Wang, Zhao, Huang, Chen and Hou57} Thus, the introduction of narrow-band-gap g-C₃N₄ and in situ N-doping will lead to the relatively stronger absorption edge in the visible light region of CsTi₂NbO₇.

FIG. 6.

Diffuse reflectance UV-visible of different samples.

The conceivable mechanism of the enhanced electrocatalytic performance on the NTCN/GCE is proposed in Fig. 7. The analogous layered structures between CsTi₂NbO₇ and g-C₃N₄ will minimize the lattice mismatch and facilitate the planar growth of g-C₃N₄ slabs over CsTi₂NbO₇ layers.^{Reference Liu, Zhu, Zhu, Dong, Hou, Xu, Chen, Xi and Hou36} Once the NTCN modified electrode was put into electric field, the CsTi₂NbO₇/g-C₃N₄ heterojunction could be excited and then produce electrogenerated electron–hole pairs. The generated electrons in the conduction band of g-C₃N₄ will move to that of N–CsTi₂NbO₇, which would react with the absorbed O₂ to yield ^•O₂⁻ on the surface of the electrode. On the other hand, the electrogenerated holes from N 2p states of N–CsTi₂NbO₇ can shift to the valence band (VB) of g-C₃N₄. Because of the enhanced active area and nanobelt structures, the nitrite can easily diffuse on the surface of CsTi₂NbO₇@g-C₃N₄ heterojunction, and then quickly oxidized by holes and ^•O₂⁻.

FIG. 7.

A schematic illustration of the possible electrocatalytic mechanism for charge transfer in the NTCN heterojunction composite.

It is well known that the pH value had a great influence on the electrochemical signals of the modified electrodes.^{Reference Zou, Yang, Bin, Wang, Li, Yang, Wang, Shiraishi and Du9} The CVs of NTCN/GCE in the different pH values were recorded in Fig. S4(a). The plot for the relationship between pH values and peak currents are shown in Fig. S4(b). It is found that the oxidation peak current is first increase and then decrease in the alkaline solution with a maximum current at pH 7.0. It was reported that the generation of N₂O at low pH value may result in decreased peak currents.^{Reference Brylev, Sarrazin, Roué and Bélanger58} Therefore, the pH 7.0 is selected as the optimal condition for our experiment.

B. Electrocatalytic behavior of NTCN/GCE toward the detection of nitrite

DPV was carried out to test the electrochemical behaviors of different modified electrodes. The DPVs of CsTi₂NbO₇/GCE, g-C₃N₄/GCE, and NTCN/GCE were conducted in N₂-saturated 0.1 mol/L PBS (pH = 7.0) containing 0.999 mmol/L nitrite, and the results are shown in Fig. 8(a). It is obvious that the NTCN/GCE shows a higher current response and a lower oxidation potential compared to CsTi₂NbO₇ and g-C₃N₄ modified electrodes. This is owing to the great synergetic effect of CsTi₂NbO₇ and g-C₃N₄ in the heterojunctions structure that provides more electron channel and shorten the diffusion distance between reactive sites. As a lower detection limit and higher sensitive electrochemical method, the DPV technology was selected for the detection of nitrite on the NTCN/GCE. The calibration curve in the range of 0.0999–3.15 mmol/L by nitrite concentration and peak current is presented in Fig. 8(b). The linear regression equation was I _pa (μA) = 4.21C (mmol/L) + 0.636 (n = 15, R = 0.9997), with a detection limit of 2.63 × 10⁻⁵ mol/L (S/N = 3). Therefore, the NTCN is an excellent sensor for sensitive detecting of NaNO₂.

FIG. 8.

(a) DPVs of CsTi₂NbO₇/GCE, g-C₃N₄/GCE, and NTCN/GCE recorded in 0.1 mol/L PBS (pH = 7.0) containing 0.999 mmol/L nitrite. (b) DPV curves of NTCN/GCE in 0.1 mol/L PBS (pH = 7.0) containing different concentrations of nitrite. The inset is the linear relationship between peak current and the concentration of nitrite.

C. Stability, reproducibility, and interference of NTCN/GCE

The stability of NTCN/GCE was investigated by successively DPV detections. From Fig. 9(a), the oxidation peak current for ten determinations of 2.85 mmol/L nitrite is nearly constant, with a relative standard deviation (RSD) of 0.67%. Besides, the prepared NTCN/GCE was used to detect nitrite (0.558 mmol/L) for the long-time stability tests. As shown in Fig. 9(b), in the first three days, the decrease of the peak current is 0.76%. And the current response is retained 96.7% for ten days later. These results indicate that the stability of NTCN/GCE is good enough for continuous analysis. The outstanding stability of the NTCN electrode can be attributed to the unique core–shell structure that reinforced the tight accumulation between layered CsTi₂NbO₇ and g-C₃N₄, which lead to powerful acting force for building the stable structure. In addition, its reproducibility is evaluated with independent electrodes [Fig. 9(c)]. When five parallel sensors are applied for detecting nitrite (2.85 mmol/L), the RSD is 2.3%, suggesting the NTCN/GCE is a reliable nanocomposite for fabricating sensors. To detect the selectivity of the sensor, the interfaces for the detection of nitrite is measured by adding possible coexisting substances into 0.1 mol/L PBS (pH = 7.0) containing 0.199 mmol/L nitrite. As shown in Fig. 9(d), the concentrations of NaCl, K₂CO₃, Li₂SO₄, glucose, AA, and uric acid (UA) are 10 times than that of nitrite, but negligible effects are observed, with RSD values of 4.30, 1.32, 2.01, 3.19, 3.46, and 0.56%, respectively. It is demonstrated that the biosensor with strong anti-interface ability is effective and dependable for daily operation.

FIG. 9.

(a) The oxidation peak currents for ten successive determinations of 2.85 mmol/L nitrite at NTCN/GCE in 0.1 mol/L PBS (pH = 7.0). (b) The oxidation peak currents of 2.85 mmol/L nitrite detected at different storage times. (c) The oxidation peak currents of 2.85 mmol/L nitrite detected at different electrodes. (d) The oxidation peak currents of 0.199 mmol/L nitrite detected with 10-fold interference species of NaCl, K₂CO₃, Li₂SO₄, glucose, AA, and UA.

D. Real sample analysis

To examine the feasibility and accuracy of NTCN/GCE, the fabricated sensor is used to analyze the concentration of nitrite in waste water by standard addition technology. The water samples are first filtered for removing particle materials and then diluted with 0.1 mol/L PBS (pH = 7.0) for five times. As shown in Table I, the recovery range is from 96.9% to 103.0%, and the RSD are both lower than 1.5%. The results indicate that this biosensor can afford high precision for practical application.

TABLE I.

Determination of nitrite in waste water.