A novel highly selective electrochemical chlorobenzene sensor based on ternary oxide RuO2/ZnO/TiO2 nanocomposites

Abstract

A novel electrochemical (EC) chlorobenzene (CBZ) sensor was fabricated using a ternary oxide RuO₂/ZnO/TiO₂ nanocomposite (NC)-decorated glassy carbon electrode (GCE). The nanoparticles (NPs) were synthesized by a wet-chemical method and characterized by X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), and ultraviolet-visible (UV-vis) spectroscopy. The synthesized RuO₂/ZnO/TiO₂ NC was layered as thin film on a GCE with Nafion (5% suspension in ethanol) adhesive, and the as-prepared sensor was subjected to CBZ analysis using an electrochemical approach. The calibration of the proposed CBZ sensor was executed with a linear relation of current versus concentration of CBZs known as the calibration curve. The sensitivity (32.02 μA μM^-1 cm^-2) of the CBZ sensor was calculated from the slope of the calibration curve by considering the active surface area of the GCE (0.0316 cm²). The lower detection limit (LD; 98.70 ± 4.90 pM) was also calculated at a signal-to-noise ratio of 3. Besides these, the response current followed a linear relationship with the concentration of chlorobenzene and the linear dynamic range (LDR) was denoted in the range of 0.1 nM to 1.0 μM. Moreover, the CBZ sensor was found to exhibit good reproducibility, reliability, stability, and fast response time. Finally, the sensing mechanism was also discussed with the energy-band theory of ternary doped semiconductor materials. The sensing activity of the proposed sensor was significantly enhanced due to the combined result of depletion layer formation at the heterojunction of RuO₂/ZnO/TiO₂ NCs as well as the activity of RuO₂ NPs as oxidation catalysts. The proposed CBZ sensor probe based on ternary oxide RuO₂/ZnO/TiO₂ NCs was developed with significant analytical parameters for practical application in monitoring the environmental pollutants of CBZs for the safety of environmental fields on a large scale.

Fig. 1. Comparison of X-ray diffraction pattern of synthesized NPs. (a) XRD patterns of TiO₂ NPs, (b) the X-ray diffracted peaks of RuO₂/ZnO/TiO₂ NCs, and (c) resulted X-ray lines of ZnO NPs.

Fig. 2. XPS survey spectrum of RuO₂/ZnO/TiO₂ NCs.

Fig. 3. (A) High-resolution XPS spectra of Ti 2p, (B) Zn 2p level orbital, (C) C 1s + Ru 3d, and (D) O 1s orbit of RuO₂/ZnO/TiO₂ NCs.

Fig. 4. (A and B) Low- and high-magnification FESEM images of RuO₂/ZnO/TiO₂ NPs, (C) image of EDS, (D) the elemental compositions of RuO₂/ZnO/TiO₂ NCs as weight and atomic percentages, and (E) size distribution of NPs.

Fig. 5. (A) UV-vis DRS and (B) (F(R)hν)²versus hν plot of (a) TiO₂, (b) ZnO and (c) 1 wt% ternary RuO₂/ZnO/TiO₂ NCs.

Fig. 6. Performance optimization of the RuO₂/ZnO/TiO₂/GCE-based chlorobenzene sensor. (A) Electrochemical investigation of toxic chemicals to execute the selectivity of the sensor at 0.1 μM in a buffer phase of pH 7.0, and (B) the control experiments to optimize the composition of ternary RuO₂/ZnO/TiO₂ NCs as CBZ sensing mediators.

Fig. 7. (A) Typical current–voltage responses of the fabricated sensor at different concentrations of CBZs, and (B) magnified electrochemical responses of the sensor at different CBZ concentrations in the range from 1.2 to 1.5 V.

Fig. 8. (A) The calibration of the CBZ sensor based on the ternary RuO₂/ZnO/TiO₂ NCs/GCE, and (B) the linearity of LDR based on current versus log(CBZ concentration).

Fig. 9. Performance of reliability test: (A) reliability test of the CBZ sensor at 0.1 μM, (B) response time estimation, (C) intra-day reproducibility performances, and (D) inter-day reproducibility performances.

Fig. 10. (A) Electrochemical responses of various toxic chemicals at 0.1 μM concentration in a buffer phase of pH 7.0, and (B) interference effect of other co-existing toxins in the chlorobenzene sensor.

Scheme 1. Mechanism of CBZ detection with ternary oxide RuO₂/ZnO/TiO₂ NCs by an electrochemical approach in an aqueous medium.

Fig. 11. Energy band diagram of nanocomposite systems when exposed to the CBZ solution; before (A) RuO₂/TiO₂, (D) RuO₂/ZnO, and (G) ZnO/TiO₂ and after: (B and C) RuO₂/TiO₂ (E and F) RuO₂/ZnO, and (H) ZnO/TiO₂.