Enhanced Sensing Performance of the Palladium Loaded Tin Oxide to Ethylene Gas

Abstract

Introduction As a plant hormone, ethylene (C2H4) plays a regulatory role in the process of plant growth and senescence [1]. In particular, C2H4 is a non-negligible factor in growth and storage for some fruits (e.g., bananas, kiwifruit and apple), the C2H4 emission can serve as a potential indicator for determining the maturity and freshness of fruits. For example, C2H4 is effective on immature fruit in the low concentration range of 10 ppb to 10 ppm [3]. With the fruit maturing, C2H4 production increases sharply to over 100 ppm, and excess C2H4 can cause the premature ripening and even decay of the fruits [3]. Therefore, developing a gas sensor to detect C2H4 is significant for fruit ripening and storage. However, C2H4 is generally hard to detect because of its small molecular size and lacking of polar chemical functionality [4], resulting in less research on C2H4 gas sensors so far. All kinds of gas sensitive materials based on metal oxide (e.g., SnO2, ZnO, Fe2O3, WO3 and In2O3) are widely used in various gas detection [5]. Combining good gas response of SnO2 with modification of noble metal could be an bright strategy for C2H4 gas sensor. In this work, the enhancement effect of Pd-loaded SnO2 on C2H4 sensing properties are investigated. And the C2H4 gas sensing mechanism is elaborated in this paper. Particularly, the potential applications of the Pd-loaded SnO2 C2H4 gas sensor are performed in the maturity and storage of fruits. Experiment All the chemicals used in the experiment were analytical reagents. The Pd-loaded SnO2 C2H4 gas sensor was fabricated by simple coating method. In brief, SnO2 powder (99.99% metals basis, 50–70 nm, Shanghai Aladdin Bio-Chem. Technology Co., Ltd) and Pd powder (99.9% metals basis, Sigma-Aldrich Co., Ltd) with an optimized mass ratio of 30:1, the mixture was ground in a mortar for 15 min and then mixed with deionized water at a weight ratio of 3:1 to form a paste. The Pd-loaded SnO2 sensor was fabricated by coating the prepared paste on the ceramic tube with a pair of gold electrodes. In order to improve the stability of the sensors, they were annealed at 400°C for 2 h in air atmosphere. Finally, a Ni-Cr heating wire was inserted into the tube to form an indirect-heated gas sensor. The pure SnO2 sensor was prepared by the same process. Results and Conclusions In terms of gas sensing performance, the results show that the response (11.1, Ra/Rg) of the Pd-loaded SnO2 sensor is about 3 times higher than that of pure SnO2 (only 3.5) for 100 ppm C2H4. And the response time also shortened significantly from 7 s (pure SnO2) to 1 s (Pd-loaded SnO2). Especially, the Pd-loaded SnO2 gas sensor possesses a good sensitivity (0.58 ppm-1) at low concentration C2H4 (0.05–1 ppm) with the excellent linearity (R2 = 0.9963) and low detection limit (50 ppb). Boosted C2H4 sensing properties of Pd-loaded SnO2 are attributed to the excellent adsorption and catalysis effects on the Pd nanoparticle surfaces.Importantly, various potential applications of the C2H4 gas sensor are further performed for monitoring the maturity and freshness of fruits. The real-time response curves of the gas sensor to five fresh varieties (banana, lemon, apple，pear and potato) can be seen that the Pd-loaded SnO2 gas sensor presents different responses to different fruits. The nonlinear increase of response to gradually increasing banana weight (about 25 g, 50 g, 75 g, 100 g and 125 g) indicates that C2H4 emissions are mutually reinforcing. To further support potential applications in fruit storage, the variation in response to one banana and one lemon at different ripening stages are presented. The results show that the response of banana changed significantly from non-climacteric to climacteric, while that of lemon changed smoothly. As a respiratory climacteric fruit, banana is susceptible to the influence of C2H4 concentration in senescence and metamorphosis, while the non-respiration climacteric lemon is more resistant to storage than banana due to the slow metabolism of lemon, which is consistent with the experimental results. In short, the above tests suggest that the potential applications of Pd-loaded SnO2 gas sensor in fruits quality evaluation like a smart nose.