Natural antioxidant functionalization for fabricating ambient-stable black phosphorus nanosheets toward enhancing flame retardancy and toxic gases suppression of polyurethane

Highlights

• The ambient stability was imparted to BP nanosheets.

• The radicals quenching effect of TA was developed to reduce the CO release.

• The flame retardancy and toxic gases suppression of BP nanosheets was explored.

Abstract

Herein, as a natural antioxidant, tannin (TA) is firstly used to functionalize black phosphorous (BP) nanosheets to improve the ambient stability and toxic suppression, thus decreasing the fire hazards of polymer materials. Compared to pure BP nanosheets, higher temperature for thermal oxidation decomposition is achieved for TA-BP nanosheets, directly confirming the ambient stability of TA-BP nanosheets. Meanwhile, from high resolution TEM and XPS results, TA-BP nanosheets after being exposed at air for 10 days present well-organized crystal structure and low PO_x bonds content. Cone calorimeter results illustrate that the incorporation of 2.0 wt% TA-BP nanosheets significantly decreases the peak value of heat release rate (−56.5 %), total heat release (−43.0 %), CO₂ concentration (−57.3 %) of TPU composite. Meanwhile, with addition of low to 1.5 wt%, the release of highly-toxic CO gas is significantly suppressed, confirmed by lower peak value (0.52 mg/m³) and decreased total release amount (−55.1 %). The obviously enlarged tensile strength (36.7 MPa) and desirable elongation at break (622 %) are also observed. This strategy not only firstly adopts bio-based antioxidant to impart excellent environmental stability for BP nanosheets, but also promotes the promising potentials of BP nanosheets in the fire safety application of polymer composites.

Introduction

As an emerging two-dimensional nanomaterial, black phosphorous (BP) nanosheets is gradually receiving considerable attention in academic and industrial fields (Tan et al., 2017). Compared the zero-bandgap of graphene, the layer-dependent direct bandgap and high carrier mobility as well as large on/off ratio make few-layer BP a desirable candidate in information storage, chemical/biosensing, and optoelectronics (Wu et al., 2018a; Zheng et al., 2017). In addition, few-layer BP also presents potential application in flexible electronics, due to high aspect ratio and excellent mechanical strength, as well as electronic conductivity (Zhu et al., 2015). Lots of research works have been devoted to develop promising potential of BP nanosheets, thus promoting the advances in technology.

Compared to desirable stability of graphene and other 2D nanomaterials exposing to oxygen and water, the most crucial issue associated BP is the poor ambient stability (Zhou et al., 2016; Favron et al., 2015; Yu et al., 2015). Based on previous literatures, degradation mechanism of BP in ambient conditions is divided into three steps: firstly, the oxygen molecules adsorbed onto the surface of BP are turned into O₂⁻ by combining the free electron produced by illumination. Follow on, O₂- dissociates onto the surface of BP and forms PO bonds with phosphorous elements. Finally, through the hydrogen-bond interaction, water molecules draw the bonded O and remove P from the surface and break the top layer of black phosphorous (Zhou et al., 2016; Favron et al., 2015). Therefore, the stability of raw few-layer BP nanosheets in ambient conditions is undesirable and seriously suppresses the application space of BP, where oxygen and moisture are impossible to avoid. As far as our information goes, the currently used protection mechanism to BP nanosheets are mainly divided into two approach, including establish isolated coating and passivate the lone-pair electron reactivity. For example, Avsar et al. employed h-BN nanosheets as encapsulating layer onto the surface of BP to isolate moisture and oxygen (Avsar et al., 2015). However, the cover of unmatched inorganic nanomaterials would deteriorate the special performance of BP nanosheets. It is certain that, regarded as white graphene, h-BN nanosheets will influence the photoelectron property of BP nanosheets. Taking into account reducing the reactivity of lone-pair electrons of P to O₂⁻, Ag⁺ was adopted to spontaneously adsorbed on the BP surface via cation–π interactions, thus rendering BP more stable in air (Guo et al., 2017a). As initiator of the entire degradation process of BP, the elimination of O₂⁻ is capable of directly stopping the degradation reaction at its source. However, recent research works didn’t pay attention to the capture of O₂⁻. Therefore, a protection strategy combining the isolate and capture of O₂⁻ is urgent for BP nanosheets.

For almost a thousand years, tea culture is gradually propagated in East Asian region and drinking tea has become an indispensable part of East Asians lifestyle (Xu et al., 2017). Tea polyphenols, as the central constituent of tea, is capable of releasing hydrogen donors to scavenging superoxide radicals, thus presenting antioxidation effect (Xu et al., 2017; Luo et al., 2016). As analogue of tea polyphenols, tannic (TA) is also a naturally derived polyphenolic compound and possesses much superior oxidation resistibility (Luo et al., 2016; Ouyang et al., 2019). Compared to tea polyphenols, TA is capable of modifying the materials surface through multiple interactions, including hydrogen-bond and π-π interactions. Aim at preventing the ambient degradation of BP nanosheets at the beginning, in this work, TA is used to coat onto the surface of BP nanosheets to eliminate superoxide radicals (Glaive et al., 2017). Meanwhile, the radical quencher effect of TA is also expected to cut off the combustion reaction.

Polymer materials are now part of most of the consumer goods but also find applications in very specific domains (Cai et al., 2018; Wang et al., 2019a; Yu et al., 2019). Among them, thermoplastic polyurethane (TPU) is particularly popular due to the soft elasticity and desirable break strength, special applies to wearable electron devices (Liu et al., 2016a, b; Lan et al., 2016; Guo et al., 2017b; Shi et al., 2017). Similar to other polymer materials, high flammability gives a huge challenge for the engineering application of TPU, where demands high fire safety safeguards. Referring to previous literatures, various layered nanomaterials were used to improve the flame retardant of TPU, including graphene, MoS₂, carbon nitride, and h-BN (Shi et al., 2017; Cai et al., 2017a, b; Feng et al., 2016; Yu et al., 2016; Yang et al., 2019). BP nanosheets, consisting completely of P elements that is most efficient flame retardant element, is able to present a significant flame retardant effect that other layered nanomaterials couldn’t provide.

Herein, we report a natural antioxidant functionalization strategy to scavenge the superoxide radicals which are onto the surface of BP nanosheets, thus hindering the ambient degradation of BP nanosheets at source. As a radical quencher, meanwhile, the adopted TA is also capable of cutting off the consecutive combustion reaction to decrease the heat release. Therefore, the enhancement effects of tannin-functionalized BP nanosheets for flame retardancy and toxic gases suppression of TPU are investigated simultaneously.