Inhibited combustion of graphene paper by in situ phosphorus doping and its application for fire early-warning sensor

Highlights

• GO-PA paper is prepared by an evaporation-induced self-assembly method.

• Fire sensor based GO-PA exhibits ultrasensitive and durable warning response.

• Enhancement in fire resistance of GO is attributed to in situ phosphorus doping.

Abstract

Although traditional fire detectors are widely used in many occasions, they still show some limitations, such as slow fire response and poor resistance to severe conditions. Thus, it is urgent to design and exploit early fire-warning device with ultrafast response and flame-retardant ability to provide timely and fast alarm. Herein, graphene oxide paper functionalized with phosphoric acid (GO-PA) is fabricated by a facile and environmentally friendly water evaporation-induced self-assembly approach. Based on the flame-induced electrical resistance transformation characteristic of GO-PA, a fire-warning detector is designed to reliably monitor early fire. Fire alarm can be triggered within 0.5 s when GO-PA samples are attacked by fire. The GO-PA composite paper is more sensitive than GO-based fire sensors reported in the literature. Phosphorus atoms are in situ doped into graphene structure during flame exposure. Combustion reaction of GO is significantly retarded by the incorporated PA, thus fire duration time of GO is greatly improved. This work paves a new way to improve fire detection performance of GO-based sensors. This work provides a guide for the development of advanced fire detection and early-warning sensors to provide reliable and continuous alarm signals.

Introduction

Fire accidents not only exacerbate safety problems about human life and health, but also lead to adverse effects on atmosphere and ecological balance. Fire accidents affect people's normal life, social and economic developments, and lead to irreparable loss of physical and mental pain. Therefore, development of effective fire prevention methods is a major global challenge. The current fire prevention and mitigation methods include flame retardant, firefighting and fire detection or alarm technologies [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]].

The early fire-warning plays an important role in active prevention and controlling of fire. To prevent and mitigate the damage caused by fire, many studies about fire monitoring have been carried out based on infrared and smoke detectors. The use of fire detection sensors plays an important role in reducing fire risk of buildings. In general, smoke or infrared detector has been widely used in indoor, by detecting the smoke or light generated from the combustion of combustible materials, usually requiring a long response time of more than 100 s, especially for many cooking scenes [11]. Smoke is generally generated after ignition, and when the concentration of smoke reaches the threshold value of the detectors, they will be triggered for alarm. However, in such situation, fire may develop into full development stage, which is the greatest fire risk scenario. Although conventional fire-warning sensors have been implemented in some fields, these methods have some limitations, such as slow fire response, poor resistance to severe conditions and the limitation of application scenario. Therefore, it is particularly important to develop highly sensitive fire-alarm devices, which will provide timely and reliable fire-alarm signals. In consequence, there is an urgent need for the development of novel fire alarm which can realize fire detection and early warning [12].

Graphene-based materials with excellent chemical and physical properties have attracted increasing attention in catalytic [13], energy conversion and flame-retardant areas [[14], [15], [16]]. Commonly, graphene oxide (GO) is produced by oxidative treatment of graphite via Hummers method [17]. As a derivative or precursor of graphene, GO still retains a graphene-like layered structure. GO consists of oxidized graphene sheets and their basal surfaces are modified mostly with epoxide and hydroxyl groups, and the edges are decorated with carbonyl and carboxyl groups [[18], [19], [20], [21]].

Electrical insulation of GO is explained by the fact that the conjugating electron structure of graphene is destroyed by these oxygen functional groups during chemical oxidation. In the condition of high temperature or flame, most of functional groups on GO can be removed, repairing the conductive network to obtain excellent electrical conductivity [22,23]. This inherent property of GO has provided enormous interest for its possible application in fire-warning field. Great advances have been made in several researches based on electrical resistance transition of GO [24], which is changed from insulation to electrical conduction under fire or high temperature. Dr. Tang from Hangzhou Normal University prepared GO wide-ribbon-coated sponges to reliably detect the potential fire hazard of combustible materials by using the characteristic of temperature-induced electrical resistance transition of GO [25]. Wu et al. from South China University of Technology constructed multiple layers of GO-silicone structures on several substrates and developed as early fire-alarm sensors and fire-retardant coatings. The coating showed distinct temperature-responsive electrical resistance change and exhibited a quick flame-detection response within approximately 3 s when encountering fire [26].

Although the existing researches have made considerable achievements, pure GO paper cannot provide continuous signals due to its poor thermal stability in air [27]. GO paper can be completely burned out under air atmosphere. The work of Huang et al. has improved fire resistance of GO paper through silane-assisted assembling route [28]. Although this modified GO paper can provide an early fire-alarm signal, its preparation process is time-consuming. Therefore, there is an urgent need for developing a simple and low cost fabrication method for advanced GO-based fire response sensors.

Although graphene has excellent performances, its limitation of poor thermal oxidative stability is incontestable, due to the presence of residual oxygen functional groups and defects [29]. Our previous work found that phosphorus doping can improve thermal oxidative stability of graphene. The boron/phosphorus doping retards the oxidation of reduced graphene oxide (RGO) by forming more stable bond configuration. The doped boron/phosphorus atoms restrain carbon gasification and reduce the reactivity of active sites [30]. Inspired from the work, we proposed in-situ phosphorus doping strategy for enhancing thermal stability of graphene. GO paper containing phosphoric acid (PA) was in-situ reduced and doped by phosphorus atoms under fire, and then high electrical conductivity of graphene is repaired and its combustion reaction under air atmosphere is retarded simultaneously.

In this work, PA-modified GO paper (GO-PA) was fabricated by a simple and environmentally friendly water evaporation-assisted self-assembly method. The preparation method is facile without using any organic solvent. We propose a fire-sensor design, which is based on flame-triggered electrical resistance conversion characteristic of GO. The fire alarm circuit consists of GO-PA paper, lamp, wire and low-voltage power. The GO-PA paper exhibits extraordinary rapid flame detection response (within 0.5 s). The GO-PA papers are promising for development of advanced early fire-alarm sensors that can provide quick and continuous signals.