Facile synthesis of g-C3N4 with various morphologies for application in electrochemical detection

Abstract

In the present study, g-C₃N₄ with various morphologies was successfully synthesized via a variety of facile in situ methods. The as-prepared products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy and X-ray diffraction (XRD). The results obtained using square wave anodic stripping voltammetry (SWASV) showed that when g-C₃N₄ was applied as an electrochemical sensor, it exhibited excellent sensitivity and selectivity for the detection of heavy metal ions including Pb(ii), Cu(ii) and Hg(ii). Compared to nanoporous graphitic carbon nitride (npg-C₃N₄) and g-C₃N₄ nanosheet-modified glass carbon electrode (GCE), g-C₃N₄ successfully realized the individual and simultaneous detection of four target heavy ions for the first time. In particular, g-C₃N₄ displayed significant electrocatalytic activity towards Hg(ii) with a good sensitivity of 18.180 μA μM^-1 and 35.923 μA μM^-1 under the individual and simultaneous determination conditions, respectively. The sensitivity for simultaneous determination was almost 2 times that of the individual determination. Moreover, the fabricated electrochemical sensor showed good anti-interference, stability and repeatability; this indicated significant potential of the proposed materials for application in high-performance electrochemical sensors for the individual and simultaneous detection of heavy metal ions.

Fig. 1. (A) XRD patterns and (B) FTIR spectra of (a) g-C₃N₄, (b) npg-C₃N₄ and (c) nanosheets of g-C₃N₄.

Fig. 2. SEM images of (a) g-C₃N₄, (b) npg-C₃N₄ and (c) nanosheets of g-C₃N₄; (d) elemental mapping images and energy-dispersive X-ray spectrometer (EDS) results of g-C₃N₄; and (e) TEM images of the g-C₃N₄ nanosheets.

Fig. 3. EDS spectra of g-C₃N₄.

Fig. 4. N₂ isothermal adsorption of (a) g-C₃N₄, (b) npg-C₃N₄ and (c) npg-C₃N₄.

Fig. 5. (a) Cyclic voltammetric responses and (b) electrochemical impedance spectra of bare GCE; g-C₃N₄, npg-C₃N₄ and nanosheets of g-C₃N₄-modified GCE in an electrolyte solution of 0.1 M KCl and 5 mM K₃[Fe(CN)₆] (inset figure shows the equivalent circuit plots in the EIS data fitting). Scan rate: 100 mV s⁻¹, polarization potential: 0.25 V, frequency spectra 1–100 000 Hz.

Fig. 6. (a–c) SWASV response towards Pb(ii) detection on g-C₃N₄, npg-C₃N₄ and g-C₃N₄ nanosheet-modified GCE in an electrolyte solution of 0.1 M HAc–NaAc (pH = 5) at the deposition potential of −1.0 V for 150 s. The potential step is 5 mV, the amplitude is 20 mV, and the frequency is 25 Hz. (d) Comparison between the sensitivity versus Pb(ii) for the modified electrodes based on the different materials considered in this study.

Fig. 7. (a–c) SWASV response towards Hg(ii) detection on g-C₃N₄,npg-C₃N₄ and g-C₃N₄ nanosheet-modified GCE in an electrolyte solution of 0.1 M HAc–NaAc (pH = 5) at the deposition potential of −1.0 V for 150 s. The potential step is 5 mV, the amplitude is 20 mV, and the frequency is 25 Hz. (d) Comparison of LODs of Pb(ii), Cu(ii) and Hg(ii) on the g-C₃N₄-modified electrodes.

Fig. 8. (a–c) SWASV response toward Cu(ii) detection on g-C₃N₄, npg-C₃N₄ and g-C₃N₄ nanosheet-modified GCE in an electrolyte solution of 0.1 M HAc–NaAc (pH = 5) at the deposition potential of −1.0 V for 150 s. The potential step is 5 mV, the amplitude is 20 mV, and the frequency is 25 Hz. (d) Comparison between the sensitivities of the modified electrodes based on the different materials considered in this study towards Pb(ii), Hg(ii) and Cu(ii).

Fig. 9. (a–c) SWASV responses towards the g-C₃N₄, npg-C₃N₄ and g-C₃N₄ nanosheet-modified GCE for the simultaneous detection of Cd(ii), Pb(ii), Cu(ii) and Hg(ii) over a certain concentration range. (d) Calibration plots towards Cd(ii), Pb(ii), Cu(ii) and Hg(ii) obtained using the g-C₃N₄-modified GCE. Deposition potential: −1 V, amplitude: 20 mV, frequency: 25 Hz and vs. Ag/AgCl/KCl (3 M KCl saturated with AgCl).

Fig. 10. (a) Repeatability study of g-C₃N₄ in 0.1 M HAc–NaAc (pH = 5) containing 4 μM Pb(ii). Deposition potential: 1 V; amplitude: 25 mV; increment potential: 4 mV; frequency: 15 Hz; vs. Ag/AgCl, and (b) EIS data of the g-C₃N₄ electrode towards Pb(ii) from the 1st cycle to 10th cycle.

Fig. 11. (a–c) SWASV responses of g-C₃N₄ towards Pb(ii), Hg(ii) and Cu(ii) in real water samples diluted with 0.1 M NaAc-HAc solution (pH 5.0) in a ratio of 1 : 4 obtained by the standard addition method and the corresponding calibration plot of the peak current against the three heavy metal ion concentrations (inset).