Doped ZnO Nanorod Array for Ultra Low NO2 Sensing

NO₂ is an important marker in the human breath for asthma whose concentration is proportional to the amount of inflammation in the lungs caused by infections. NO₂ is also present in the exhaust of the vehicles and the OSHA recommends no more than 5 ppm NO₂ in air. Currently 235 million people are suffering from asthma worldwide and the number is expected to increase drastically owing to the increase in global pollution level and dependence on fossil fuel. Metal oxide semiconductors such ZnO have remarkable sensing properties for, NO₂. Sensor response (S) is a function of the resistance of the ZnO on inter-digitated Au electrodes which is given by the following equation:

S = R_air/R_{gas ,} where, R_gas is the resistance of the film in presence of testing analyte and R_air is the resistance of the film in air. The gas detection occurs because of the reaction on the ZnO surface in presence of the NO₂ molecule which causes a reduction in electron concentration and hence fluctuation in electrical resistance. This can be explained by the following equation:

NO₂ + e^- = NO⁺ + O^-_ads

Here we report synthesis of TiO₂ coated ZnO, Mg, Co, and Ag doped as well as undoped thin ZnO nanorod array directly on Au interdigitated electrodes using a low cost, environmentally friendly and large area application two-step chemical bath deposition (CBD) technique [1, 2] for selective detection of ultralow (0.1 ppm) NO₂ in humid air at 280 °C. The structure and morphology of the ZnO films prepared by CBD has been probed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The as-prepared doped and undoped ZnO sensors were tested for the detection of NO₂ (0.1-5 ppm in simulated breath) in the temperature range of 50-280 ˚C as shown in the figure 1. Characteristics such as sensitivity, selectivity, limit of detection, reproducibility, and stability will be reported. Role of dopants in enhancing the selectivity will be explained. The selectivity for the interfering analytes such as humidity, CO₂, NO₂, H₂ and ammonia will also be investigated. Finally the sensing mechanism will be discussed in detail. 

References:

[1] V. Manthina, T. Patel, A.G. Agrios, Number Density and Diameter Control of Chemical Bath Deposition of ZnO Nanorods on FTO by Forced Hydrolysis of Seed Crystals, Journal of the American Ceramic Society, 97 (2014) 1028-1034. 

[2] V. Manthina, J.P. Correa Baena, G. Liu, A.G. Agrios, ZnO–TiO2 Nanocomposite Films for High Light Harvesting Efficiency and Fast Electron Transport in Dye-Sensitized Solar Cells, The Journal of Physical Chemistry C, 116 (2012) 23864-23870.