Flexible VOCs Gas Sensors Integrated With Heating Platform On Polyimide Substrates

Nowadays,the growing demands for portable and wearable electronics have promoted increasing academic and industrial interests in flexible electronics.Gas sensors have received extensive attention due to their important roles in a range of daily life and industrial applications such as medical diagnosis,environmental monitoring,food industry,public safety,agriculture and etc..The development of a simple,low-cost,lightweight,flexible and easy-to-integrate flexible gas sensor has become an increasingly urgent need for real-time and effective monitoring of harmful gases in the environment.The difficulties to be solved in implementation of flexible wearable gas sensor include:the realization of metallization with high stability and reliability on the flexible substrate;the stability during the bending process;the high working temperature and the difficulty to integrate into flexible devices;the poor selectivity of gas sensor.In our work,in combination of the SMIE and inkjet printing technology,the double-sided metallization structures are realized.The sensors with the integrated heaters can offer twofold functions:in-situ sensitive material synthesis and elevated temperature detection of VOCs.This paper also demonstrates layer-by-layer?LBL?self-assembled titania?TiO₂?/graphene film device towards VOCs sensing at room temperature.The main work is as following:First,the double-sided metal patterning of the PI substrate was realized by using the inkjet printing technology and the dry film lithography process in combination with the surface modification and ion exchange technology,respectively.Then,a heater-interdigital electrode integrated structure was prepared.The properties of surface morphological,electrical and mechanical are studied.The results indicated that the fabricated Ag/PI offers the properties like high conductivity,flexibility and reliability,which is suitable for flexible and wearable applications.The whole processes are carried out in the solution under the low temperature?<150?C?.Secondly,the heating behavior of Ag films resistors was studied.The heaters with different patterns were prepared and tested for electrical and thermal properties.The results showed that the resistance of heating resistors was chosen.The temperature,ranging from 25?C to 280?C,can be realized by adjusting the DC bias voltage across the resistive heater.Moreover,the PI-Ag-PI sandwich structure was introduced to solve the burnout problem of heater during operation and improve the reliability of heater.The concept of flexible heating platform was proposed innovatively.Based on STM32 microcomputer and PWM technique,the heating control system with temperature range of 25-150?C.and control accuracy of±0.5°C was realized for flexible heating platform application.Thirdly,with the aid of integrated heater,the nano-leaf sized zinc oxide films were first in situ deposited on interdigitated electrodes to construct the gas sensor.The responses of the prepared flexible heated VOCs gas sensor to ethanol,acetone and isopropanol were tested,respectively.The results indicated that the heater introduction can obviously improve the sensor performance.It is found that the sensitivity increased monotonically with temperature?<150oC?.Since the gas desorption on sensor surface had been facilitated,the sensor had the shortened response and recovery time,and the initial resistance drift is also tolerable.In addition,the heating process can effectively reduce the interference from humidity during acetone detection.Fourthly,inordertofurtherimprovethesensorselectivity,ZnO-SnO₂ nanocomposites were in situ deposited on a flexible PI substrate,and the responses to ethanol,acetone and isopropanol at different temperatures were tested.By adjusting the temperature of the heater,the selectivity of gas sensor is achieved.Lastly,a more stable nickel electrode was fabricated on flexible PI substrates using SMIE technology and dry film lithography process.At the same time,a TiO₂modified graphene oxide gas sensor was fabricated by layer-by-layer self-assembly technique.The prepared sensor has achieved high sensitivity and selectivity for ethanol detection at room temperature and is suitable for wearable devices.The related technologies discussed in this paper have important reference value for the future research of wearable flexible gas sensors.