Chemical Sensors and Analytical Methods for Environmental Monitoring
2023
Milda Petruleviciene; Irena Savickaja; Jurga Juodkazyte; Arunas Ramanavicius.
Abstract
Volatile organic compounds (VOCs) are a notable group of indoor air pollutants released by household products. These substances are commonly employed as solvents in industrial operations, and some of them are recognized or suspected to be cancer-causing or mutagenic agents. Due to their high volatility, VOCs are typically present in surface waters at concentrations below a few micrograms per liter. However, in groundwater, their concentrations can reach levels up to thousands of times higher. This study analyses the applicability of the photoelectrochemical (PEC) sensing of VOCs in aqueous medium. Tungsten oxide and bismuth vanadate photoanodes were tested for PEC sensing of xylene, toluene, and methanol in sodium chloride and sodium sulfate electrolytes. The crystalline structure and morphology of coatings were analyzed using XRD and SEM analyses. Photoelectrochemical properties were evaluated using cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy. The results of the study show that aromatic compounds tend to block the surface of the photoelectrode and interfere with the PEC sensing of other substances. WO3 photoanode is found to be suitable for the PEC sensing of methanol under the mild conditions in aqueous electrolytes; however, electrode engineering and assay optimization are required to achieve better detection limits.
A2.2. Synthesis of Tungsten Oxide and Bismuth Vanadate
BiVO4 coatings were synthesized through a sol-gel process as detailed in reference [35]. Initially, 2.94 g of Bi(NO3)3 × 5H2O and 0.702 g of NH4VO3, with a molar ratio of 1:1, were dissolved in 23% HNO3. Subsequently, 2.52 g of citric acid (C6H8O7) was added under continuous stirring, resulting in the formation of a transparent blue solution. To adjust the viscosity of the sol-gel, 1 g of polyvinyl alcohol (PVA) and 3 mL of acetic acid were introduced. After 4 h of mixing, 0.702 g of (NH4)6Mo7O24 × 7H2O, corresponding to 10 atomic percent of Mo, was added to the solution, and the resulting mixture was stirred overnight (12 h) on a magnetic stirrer at 20 °C. The obtained sol-gel was then utilized for the deposition of thin films on FTO substrates. The FTO slides were prepared using the same method as in the synthesis of tungsten oxide. The procedure for dip-coating was conducted with a dip-coater (Nadetech, ND-DC 11/1) at an immersing and pulling rate of 100 mm/min, lasting for 60 s. After the deposition, the layers were annealed in air at 450 °C (with a heating rate of 1 °C min−1) to achieve crystalline Mo-doped BiVO4 coatings. The deposition process was repeated twice.
Keywords: Photoelectrochemistry; Volatile organic compounds; Photoanode; Tungsten oxide; Bismuth vanadate; Detection.