Frequency-tuned selectivity enhancement of Mg@ZnO-TiO₂ nanoflake-based heterojunction sensor devices

Selectivity improvement by the frequency tuning of Mg@ZnO-TiO₂ nanoflake-based heterojunction devices under exposure to different volatile organic compounds is the prime focus of the present paper. The synthesis of Mg@ZnO-TiO₂ nanoflakes was carried out using a solution process followed by a low-cost hydrothermal method. A capacitive measurement approach was used to find the resonant frequencies of the device in 2-butanone, acetone, 2-propanol, ethanol, and methanol environments in the frequency range from 0.001 to 220 kHz. A Cole-Cole plot derived from impedance measurements suggested that the device impedance consisted of capacitance (C_z = 2.01 pF and C_T = 2.17 pF) and resistance (R_z = 13 552.2 kΩ and R_T = 3500.574 kΩ) from the ZnO nanoflake layer and TiO₂ thin film layer, respectively. The maximum capacitive responses to 2-butanone (C₄H₈O), acetone (C₃H₆O), 2-propanol (C₃H₈O), ethanol (C₂H₅OH), and methanol (CH₃OH) were recorded at resonant frequencies of 1.000 kHz, 0.791 kHz, 0.702 kHz, 0.319 kHz and 0.103 kHz, respectively, at the corresponding temperature of 125 °C, 100 °C, 100 °C, 75 °C, and 75 °C, respectively. The device offered optimum capacitive responses of 221.32%, 242.65%, 317.09%, 373.96%, and 401.24% to 100 ppm of 2-butanone, acetone, 2-propanol, ethanol, and methanol, respectively. Experimental observation confirmed that the capacitive response inversely varied with the resonant frequency. Such an inverse relation was correlated with the dielectric variation at the junction interface, change in the molecular weights of VOCs, and their sticking coefficient. An equivalent circuit diagram with the help of adsorption-desorption isotherms and an energy band model is illustrated to correlate the device optimum capacitive response at the tuned frequencies under different VOC media.