Variation of Sputtered WO3 Film Thickness in Ag (NPs)/WO3/Au (NPs) System for Optimizing Sensing Behaviors to NH3

Abstract

Introduction Ammonia (NH3) is one of the major air pollutants. It can cause serious damage to the eyes, skin, and respiratory system when its concentration exceeds 300 ppm [1]. Therefore, it is necessary to develop an efficient NH3 gas sensor to monitor its concentrations in the environment and varies industrial sectors. So far, several nanostructured materials have been investigated to detect NH3 gas, such as ZnO, SnO2, and WO3 [2]. However, these materials usually suffer from a lack of sensitivity, especially at low operating temperatures. Several strategies have been applied in the literature to improve the NH3 sensing performance, including surface treatment, doping with other materials, and forming a heterojunction. Furthermore, it is generally accepted that the decoration of the thin films will noble metals such as Au, Pt, or Pd will improve the sensor response owing to the increased depletion layer thickness of gas sensing materials and additional electrical resistance changes due to the built-in barrier high formed at noble metal nanoparticles and sensing material junctions. In our previous studies, we developed a method to decorate oxide thin-film surfaces with well-dispersed and uniform noble metal nanoparticles and tested their gas sensing performance toward different gases such as [3-4]. Additionally, the flow of charge carriers between the metal oxide and the attached nanoparticles was found to depend on the work function difference between them. For instance, silver (work function = 4.33 eV), or gold (work function = 5.1 eV) decorated metal oxide has a different impact on the gas sensing properties. The flow of electrons between metal oxides such as WO3 and the attached silver, or gold nanoparticles has two opposite directions: one from metal oxide to the Au and the second one from Ag to the metal oxide owing to the work function difference between Ag/ metal oxide, and Au/metal oxide. This work intends to fabricate double side noble metals activated sputtered WO3 thin film and investigate the effect of the WO3 thickness on the NH3 gas sensing properties. Method Ag (NPs)/WO3/Au (NPs) films were fabricated by DC sputtering followed by post annealing treatment in nitrogen atmosphere. The deposition of the Ag (NPs)/WO3/Au (NPs) films was carried out into three steps. First, Ag thin film was deposited for a period of 30 s by applying 20W. Afterward, WO3 films having a different thickness ranging from 5nm to 150 nm were deposited via DC reactive sputtering at 100 W in oxygen. In the third step, Au was sputtered using DC sputtering at 25 W for 30 s. Subsequently, the samples were heated at 600 ⁰C in a nitrogen atmosphere for 3 h to convert the Ag, and Au layer to nanoparticles. Different analytical techniques were used to study the physical and the chemical properties of the fabricated films. Results and Conclusions Figure 1(a) shows the FESEM image of the Ag (NPs)/WO3/Au (NPs) film. As can be observed, two different size distributions of nanoparticles can be seen which could be ascribed to the formation of Ag and Au nanoparticles. The formation of WO3 film, Ag and Au nanoparticles was also confirmed using FESEM-EDX mapping (not shown). Gas sensing results showed Ag (NPs)/WO3/Au (NPs) sensor with a thickness of 30 nm demonstrated the highest gas response to NH3. Fig. 1(b) shows the dynamic response-recovery curves of the Ag (NPs)/WO3/Au (NPs) sensor for different concentrations of NH3. It can be noticed that all the sensing responses increased with the increase of NH3 concentration. The repeatability of the fabricated sensor was implemented up to five cycles in 100 ppm. As can be observed, the developed sensor displayed high repeatability under the tested conditions. Figure 1(c) shows the five-cyclic response of the fabricated sensor towards 100 ppm NH3 at 250 ⁰C. As can be noticed, the sensor capable of complete recovery upon removal of NH3. The deviation in the sensing response was less than 2% thus ensuring excellent repeatability. The long-term stability of a sensor is of great importance for almost all practical applications. To investigate the long-term stability of the sensor, the gas sensitivity was recorded at 100 ppm NH3 for over 20 days and the obtained sensing results plotted in Figure 1 (d). The response of the fabricated sensor decreased by about 5% in comparison to its original characteristic, confirming its excellent stability.