High-performance and wearable hazardous gases sensor based on n-n heterojunction film of NGO and tetrakis(1-pyrenyl)porphyrin

Highlights

• NGO/CoTPyrP flexible film is fabricated by QLS method for the first time.

• The NGO/CoTPyrP based gas sensor showed superior sensitivity for four noxious gases.

• The compact heterojunction film displayed excellent mechanical flexibility.

• The sensing performance of the device remains stable in a wide humidity range.

• The remarkable synergy of NGO/CoTPyrP enhanced gas sensing properties.

Abstract

The popularity of “Internet of Things” and portable electronic devices creates unprecedented demands for wearable gas sensors with excellent performance. In this study, the flexible n-n heterojunction film is firstly produced from metalloporphyrin complex 5,10,15,20-tetrakis(1-pyrenyl) porphyrin cobalt (II) (CoTPyrP) and nitrogen-doped graphene oxide (NGO) film, using solution-processing quasi-Langmuir–Shäfer (QLS) method and employed as the electrochemical identification layer for the wearable sensor. Thanks to the attractive electron-transfer properties from porphyrin to NGO, and the local regulation of electron transport by N and C atoms with different electronegativity on NGO, the resulting sensor shows good responses to NO₂, SO₂, NH₃, H₂S gases with the low detection limit (LOD) of 6, 74, 113 and 178 ppb, respectively. The uniform and compact structure of the heterojunction films provide excellent mechanical flexibility and suppress the penetration of gases into the film to obtain fast recovery speed. In addition, a sensor array consisting of NGO/CoTPyrP heterojunction and CoTPyrP film sensor is established, achieving selective identification of four hazardous gases. The present work provides potential application for hazardous gases identification in actual systems, and proposes an effective method to develop new flexible n-n heterojunctions for wearable gas sensors.

Introduction

According to IC Insights’ new 2020O-S-D Report ((A Market Analysis and Forecast for Optoelectronics, 2020) http://www.icinsights.com/services/osd-report/report-contents), the sale of sensors and actuators hit a record high in 2019, reaching $86.2 billion, and marking its 10th consecutive year of increasing sales. The report showed that by 2024, with the popularization of the Internet of Things (IoT), the global sales of sensors and actuators would be increased by a compound annual growth rate of about 4%. More and more sensors and actuators are integrated into new high-capacity applications, such as the millions of internet devices in the IoT and other flexible equipment. Therefore, the development of advanced sensors and actuators employing new materials, designs and specific process engineering for the devices is an important requirement.

Compared to conventional non-flexible sensors, the flexible sensors that can captured target analytes efficiently and generate higher quality signals, have attracted much more attention, due to their potential to integrate into a wearable platform for real-time health monitoring and safety warning anytime and anywhere (Li et al., 2018, Kano et al., 2017, Güder et al., 2016, Zhu et al., 2019b, Zhu et al., 2019a). On the other hand, the preparation of flexible sensors requires novel material design methods, including active materials and conductors, and the choice of flexible substrates (Choi et al., 2016). For flexible materials, π-conjugated, print-compatible, solventable and lightweight organic semiconductors (OSCs) are better choices than metal oxides such as In₂O₃ (Liu et al., 2017), CuO (Zhou et al., 2018), Fe₂O₃ (Pistone et al., 2013) and SnO₂ (Zeng et al., 2019), due to their effective interactions with target gases through the formation of charge-transfer complexes and non-covalent bonding (hydrogen, dipole-dipole, and Van der Waals bonding) even at room temperature (Liu et al., 2019a, Liu et al., 2019b, Liu et al., 2019c, Lu et al., 2018, Wu et al., 2018, Wang et al., 2016, Wang et al., 2017). Among them, porphyrins, both metal (MPors) and metal-free(H₂Pors), as the typical OSCs, have been considered as ideal choices for the development of wearable gas sensors due to their mechanical flexibility, easily adjustable chemical properties via regulation of the metal center and substitution of functional groups on the porphyrin macrocycle (Zhu et al., 2019b, Zhu et al., 2019a, Singh et al., 2017). However, the porphyrins have two major drawbacks for application as gas sensors: uncontrollable aggregation microstructures for the solid films and relative low film-conductivity, which restrain the analyte-porphyrin interaction and the transducing functions of the sensors (Wang et al., 2017). Many researchers have attempted to overcome the disadvantages to increase sensing performance by introducing guest groups to enhance the binding sites on the film (Liu et al., 2019a, Liu et al., 2019b, Liu et al., 2019c) and constructing heterojunction to improve solid films conductivity (Yu et al., 2016). The introduction of guests increases the production cost and difficulty of sensors. Porphyrin-based heterojunction thin films with suitable conductivity and easily accessible active sites to achieve better flexible gas sensor performance are highly desirable but remain challenging to date.

Herein, a flexible bilayer n-n heterojunction as a chemiresistive sensor for sensitive detecting NO₂, SO₂, NH₃ and H₂S gases were fabricated based on n-type 5,10,15,20-tetrakis(1-pyrenyl) porphyrin cobalt (II) (CoTPyrP) as a top layer and n-type nitrogen-doped graphene oxide (NGO) as a sub-layer, deposited it on PET (polyethylene terephthalate) substrates by using the solution-based quasi-Langmuir–Shäfer (QLS) technique (Chen et al., 2010), and as shown in Scheme 1. The fabricated NGO/CoTPyrP-based gas sensor showed significantly improved sensitivity and very low detection limits. Moreover, the NGO/CoTPyrP sensor exhibits excellent flexibility without visible degradation in performance even after 500 bending cycles owing to the uniformly dense structure of the bilayer films. The present sensor possessing the advantages of sensitive response, low power consumption, mechanical flexibility and room temperature operation, was expected to integrate into smart wearable systems for real-time environmental monitoring.