High Performance UV-LED Activated Sensor Development By Modulating the Morphology and Composition
Authors: Espid Ehsan , Adeli Babak , Taghipour Fariborz 
Published: May 2020

Metal Oxide Semiconductor (MOS) gas sensors have been studied for monitoring indoor/outdoor air pollutants. A drawback of conventional chemical-resistive gas sensing is the high operating temperature (up to 500°C) that is required for surface reactions. The high temperature operation, however, causes drift problems and makes sensors unsafe for environments containing flammable gases.

Ultraviolet light-emitting diode (UV-LED) is a new technological field that has recently seen rapid improvements and novel possible applications. These include the development of UV-LED based gas sensors. UV photons emitted from UV-LED could activate sensors to allow detection of chemicals at ambient temperature. UV irradiation can also improve the sensor performance in harsh environments, where SO2 or NOx gases are present, through desorbing surface contaminants. The state-of-the-art ultraviolet light emitting diode (UV-LED) that emit radiation at a power of a few milli-watts is a promising UV source for this application.

UV-activated sensors have a variety of advantages, compared to the traditional chemi-resistive metal oxide semiconductor (MOS) sensors, such as higher stability, lower preparation time, and the ability to safely detect flammable gases. The advances in ultra-violet light emitting diode (UV-LED) technology and its utilization in the gas sensing industry may offer a breakthrough in UV-based sensors, providing several important progresses such as broader application, smaller size, and higher efficiency of the developed sensors.

Among the various metal oxides that have been studied for gas sensing applications, ZnO nano-materials have shown promisebecause of high electron conductivity features. ZnO has strong luminescent properties, broad UV absorption, and high stability. Thephoto-activity of ZnO can be improved by synthesizing one-dimensional (1D) nanostructuresand by functionalizing with catalytic metal nanoparticles. This combination lowers the requiredactivationenergy and reduces the recombination rate of photo-generated charge carriers.

We will present the fabrication of a sensitive 1D nanostructure ZnO gas sensor decorated with Pt nanoparticles to detect low concentrations of toxic gases at room temperature under UV-LED irradiation (Figure 1). ZnO nanoparticles and nanowires decorated with platinum nanoparticles in different loading concentrations were prepared to detect low concentrations of NO2 under UV-LED irradiation. Solution precipitation, self-assembly crystallization, and photo-deposition techniques were used to synthesize the sensing material. The synthesized sensors at different stages of development were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) analyses.To obtain a certain visual evidence of Pt nanoparticleson the surface of ZnO nanowires, High-resolution transmission electron microscopy (HRTEM) and STEM imaging were employed. The presence of Pt nanoparticles was further confirmed through high-angle annular dark-field (HAADF) STEM analysis.

Although the sensing response of the pristine ZnO nanoparticles was relatively high (0.4) for NO2 detection, the sensing response improved significantly using ZnO nanowires in the identical photo-activation settings. The decoration ofthe surface of the ZnO nanowires with Pt nanoparticles further enhanced the sensing performance, whereas the 0.1wt% Pt-decorated ZnO nanowire sensor exhibited a response of more than one order of magnitude higher (1.5) than the response generated by the ZnO nanoparticles. This improved performance is attributed to the role of Pt active sites in promoting NO2 adsorption on the ZnO nanowires’ surface as well as enhancing the layer electron utilization.

We will present the results and discuss how UV-LED has the potential to significantly impact sensor development and portable gas monitoring systems by enabling novel sensor design concepts.

Figure 1