染料敏化半导体涂层的室温NO2气敏特性及响应机理研究

Semiconductor gas sensors for environmental monitoring have the features of low cost, small size, high sensitivity, etc. Room temperature semiconductor gas sensors may further reduce the cost, and improve the stability and safety. However, at room temperature, low surface activity of semiconductor sensitive layer results in low chemical reactivity, slow reaction rate and long response time, which is a challenge for the development of room temperature gas sensors. In this project, the role of dye in dye-sensitized solar cells is extended to the application of semiconductor gas sensors, to develop room temperature sensors with an assistance of visible-light illumination. The sensitive layers will be prepared by sensitizing tungsten oxide (WO3) and zinc oxide (ZnO) coatings with N719 dye. The dye sensitized semiconductor coatings will be submitted to oxygen adsorption-desorption tests in the dark and under light illumination. The oxygen adsorption-desorption tests will be conducted through measuring electrical resistance and Quartz Crystal Microbalance (QCM) frequency responses of the sensitive layers in the presence or absence of oxygen in nitrogen. The improvement of sensing performance of dye sensitized sensors to nitrogen dioxide (NO2) will be characterized. The roles of dye sensitizer and light illumination involved in the enhancement of NO2 sensing properties of the sensors will be elucidated. This project will help us to understand the critical problems existing in sensing application based on dye sensitized semiconductors. It will also be of importance for developing other type room temperature gas sensors.

半导体环境气体传感器具有成本低、体积小、灵敏度高等特征，降低传感器工作温度到室温能进一步提高经济性、稳定性和安全性，然而半导体气敏层在室温下表面化学活性低、反应速率慢、导致传感器灵敏度低和响应时间长，成为室温气体传感器亟待解决的重要问题。本项目依据染料敏化太阳能电池中染料敏化半导体的原理，拟采用N719染料敏化氧化钨（WO3）或氧化锌（ZnO）气敏层，借助于可见光照射增强传感器在室温时对二氧化氮（NO2）的响应能力；采用石英晶体微天平，结合气敏层电阻实时测量，研究在特定波长光照和黑暗两种条件下氧分子在传感器表面的吸附-解吸附行为和NO2的化学反应规律；研究传感器在可见光照射下对低浓度NO2气体的响应特性和气敏机理，阐明染料和可见光在传感器表面化学反应中的增强机制。本项目的研究结果可为染料敏化半导体在室温NO2气体传感器领域的应用提供理论支持，对于研究其它类型室温气体传感器也具有指导意义。

二氧化氮（NO2）是一种影响空气质量的重要污染物，利用室温气体传感器检测NO2浓度，不仅意味着较低的能耗，更简单的传感器结构，也意味着较高的材料稳定性及输出稳定性。然而传感器在室温下表面化学活性低、反应速率慢、导致传感器灵敏度低和响应时间长，成为亟待解决的重要问题。本项目在国内外研究基础上，针对室温反应速率慢的突出问题，以氧化锌（ZnO）为研究对象，利用染料敏化、窄带隙半导体和表面氧缺陷等方式，提高气体传感器在室温下的气敏性能并研究气敏增强机制。主要研究内容包括：（1）通过N719染料敏化非晶ZnO薄膜开发常温NO2气体传感器。N719染料敏化传感器对1.25-10 ppm NO2在室温下显示出良好的响应，且传感器响应随着NO2浓度的增加而线性增加。发现气体湿度对染料敏化传感器的基线电阻有重要影响，QCM结果证实电阻变化是水分子在ZnO膜上非饱和吸附的结果；（2）利用窄带隙半导体硫化镉（CdS）对ZnO涂层进行敏化，利用CdS吸收可见光，将电子转移到ZnO表面，实现CdS-ZnO在室温下对NO2气体的响应；（3）通过溶液前驱体等离子喷涂（SPPS）制备具有氧缺陷的ZnO涂层，传感器的禁带宽度由于高浓度氧缺陷的存在而降低，ZnO涂层的光吸收范围由于价带和导带中形成的深施主能级从紫外光区扩展至可见光区，传感器对NO2表现出明显的响应，证实气敏增强机制是由于涂层的高浓度氧缺陷和高孔隙结构；（4）通过将ZnO涂层浸泡在H2O2溶液中并热处理制备氧缺陷ZnO涂层，测定涂层氧缺陷的浓度，发现随氧缺陷的浓度升高，传感器对可见光的响应范围扩大，禁带宽度减小；利用第一性原理计算获得氧缺陷对禁带宽度的影响规律；气敏测试的结果显示，随着氧缺陷浓度的升高，传感器的电阻值降低，其响应及响应动力学性能提升。