DETECTION MECHANISM STUDY ON ZINC OXIDE BASED FRUIT RIPENESS SENSOR

• Main Author: Dzaudan Naufal, Farrel
• Format: Final Project
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/66734
• Institution: Institut Teknologi Bandung

Fruit ripeness detection has attracted much attention to solve post-harvest loss by overripening and to develop smart farming based on IoT (internet of things). But the fruit ripeness detection methods, done by detecting the ethylene gas which indicates fruit ripeness, are still unable to be implemented effectively for fruit monitoring. Ethylene detection technologies based on metal oxide chemoresistive sensors are the readiest method to be implemented in large-scale fruit monitoring for their simple system, small size, the ability for continuous measurement, low cost, and easy integration with electronic systems and computers. Yet, these technologies still suffer from high working temperatures and low selectivity. Recent studies have succeeded in developing room temperature chemoresistive sensors based on ZnO-Ag metal oxide with good selectivity. This breakthrough shows that metal oxides are able to become high-performance sensing materials for ethylene detection with the right engineering methods. To choose the right metal oxides and the right processing methods, the reaction mechanism which governs the sensing material performance must be known first. However, the reaction mechanisms of ethylene gas on metal oxide surfaces are still poorly understood, and the governing mechanisms which determines the ethylene sensing ability are also yet to be known. In this undergraduate thesis, the reaction mechanism of ethylene gas on zinc oxide surface is studied by a combination of Density Functional Theory calculation and microkinetic simulation. Reaction mechanisms built from the ionosorption model, ethylene oxidation to ethylene oxide through Eley-Rideal and Langmuir-Hinshelwood mechanisms, oxygen vacancy formation through Mars-van Krevelen mechanism, and oxygen vacancy diffusion are considered to predict sensor response without conducting any experiment. Calculation results show that ethylene adsorption cannot give a change of surface charge by a significant amount, while oxygen subtracts a considerable amount of surface charge. Furthermore, ethylene gas concentration affects the number of adsorbed oxygen species at the surface, giving a response of conductivity change that correlates to the ethylene gas concentration. This shows that the detection mechanism of ethylene gas on zinc oxide follows the ionosorption model, and the main reaction that gives ethylene gas sensing ability of the sensor is ethylene oxidation to ethylene oxide. Moreover, results show two important reaction steps that must exist in the reaction mechanism to give recovery ability of the sensor: (1) oxygen vacancy formation through Mars-van Krevelen Mechanism, and (2) oxygen vacancy diffusion. Simulation results on several models of ethylene gas detection mechanism show that reaction mechanisms build from the ionosorption model, ethylene oxidation to ethylene oxide through Langmuir-Hinshelwood mechanism, oxygen vacancy formation through Mars-van Krevelen mechanism, and oxygen vacancy diffusion (LH-MvK-OVD model) give an optimal working temperature of ~650 K and good linear response to ethylene gas concentration. The aforementioned model gives a sensor’s performance prediction that agrees well with experimental results. This reaction mechanism model could be developed to predict optimal working temperature and response linearity to ethylene gas concentration of other sensing materials. On the other hand, the LH-MvK-OVD model shows that the optimum working temperature of the sensor could be affected by the reaction activation energy of three reaction steps: (1) oxygen dissociation, (2) ethylene oxidation to ethylene oxide through Langmuir-Hinshelwood mechanism, and (3) oxygen vacancy diffusion. The LH-MvK-OVD model predicts a low optimum working temperature if the oxygen vacancy diffusion activation energy is low enough. This suggests that to achieve an ethylene gas sensor with a low working temperature, a low reaction activation energy of the three aforementioned reaction steps must be reached by choosing the right sensing material and engineering methods of the sensing materials.