Enhancement of sensitivity of silica photonic crystals to carbon dioxide by APTES modification

Abstract

The photonic band gap (PBG) of photonic crystals (PCs) has been proven to move in different gas atmospheres. According to this characteristic, silica PCs were used for the detection of different gases with low concentrations from 1 ppb to 100 ppm in our previous work. (3-aminopropyl)-triethoxysilane (APTES) was used as a modifier for the surface modification of SiO₂ colloidal spheres, and the amino groups were grafted onto the surface of the colloidal spheres by chemical grafting. In this work, the influence of modifier dosage on gas sensitivity and optical properties was discussed. The results suggest that the PCs modified with APTES has improved the gas-sensitivity of CO₂. It was worth noting that the PCs before and after modification have different adsorption behaviors under single gas and mixed gas atmospheres.

Introduction

As one of the constituent components of the atmosphere, CO₂ is abundant in nature [1]. Although the low concentration of CO₂ is not harmful to humans, for some specific industries, the concentration of CO₂ plays a decisive role. For example, with the acceleration of global industrialization and the popularization of miniaturized production equipment, the CO₂ concentration level in the equipment affects the quality of products, especially for modern animal husbandry [[2], [3], [4], [5]], greenhouse agriculture [[6], [7], [8]] and manufacturing [[9], [10], [11]]. Therefore, it is of great significance to study low concentration CO₂ detection devices.

As a periodic dielectric structure, photonic crystals (PCs) have the unique photonic band gap (PBG) that is very sensitive to change in external conditions. Therefore, PCs can be used as gas sensing materials, including polymer and inorganic non-metal oxide PCs [[12], [13], [14], [15], [16], [17]]. When the PCs exposure to the detecting gas, the inherent refractive index of PCs will change, resulting in the PBG of PCs shift which can be employed to identify the gas. Hong [18] used amino-functionalized inverse opal PMMA film to distinguish CO₂ through the full-color response of a small amount of gas samples in the entire concentration range, which provided a simple, sensitive, quantitative and anti-interference for CO₂ monitoring method. Lin [19] prepared three-dimensional macroporous PCs functionalized with amino groups, which could react with CO₂ in the presence of humidity. The adsorption of CO₂ caused the red shift and amplitude reduction of the PBG, thus achieving visual CO₂ detection. The above shows that the appointment of amino can improve the CO₂ gas-sensing performance of sensing materials. However, the detecting concentration of CO₂ in these reports is high (more than 10000 ppm), and there are few reports about detection of low concentration CO₂ (1 ppb-100 ppm).

In this work, SiO₂ colloidal spheres were prepared by using StÖber method [20]. (3-aminopropyl)-triethoxysilane (APTES) was employed to modify the surface of SiO₂ colloidal spheres [[21], [22], [23], [24]]. The SiO₂ PCs films were obtained by the vertical deposition method [25,26]. The single CO₂ gas and binary mixed gas with low concentration from 1 ppb to 100 ppm were used as detecting gases. The effect of APTES modification on the sensing performance of SiO₂ to CO₂ was investigated.