Natural antioxidant functionalization for fabricating ambient-stable black phosphorus nanosheets toward enhancing flame retardancy and toxic gases suppression of polyurethane
Graphical abstract
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.
Section snippets
Raw materials
Tannin, N,N dimethylformamide (DMF, AP), red phosphorous (98.5 %) were supported by Aladdin Industrial Co., Ltd. (Shanghai, China) and used directly without further purification. Thermoplastic polyurethane (TPU) resin was obtained from Bangtai Material Co., Ltd., China.
Preparation of TA-functionalized BP nanosheets (TA-BP)
Bulk BP crystals were fabricated by vapor phase growth using red phosphorus as precursor, based on previous literatures (Lange et al., 2007; Baba et al., 1989). After being obtained, 0.5 g bulk BP crystals were grinded to fine
Characterization of TA-BP nanosheets
Tannin (TA), one of the most common tea polyphenols, is a superoxide radical scavenger that is capable of modifying the surface of the BP nanosheets to protect it from being attacked by superoxide radicals. The procedure for preparing TA-BP nanosheets is shown in Fig. 1. The sonication exfoliation of BP nanosheets is carried out in ethyl alcohol which contained excess TA. The coupling effect between π electrons of TA and lone pair electrons of BP nanosheets is the driving force of surface
Conclusion
The functionalization and surface protection of BP nanosheets were successfully achieved with the use of natural antioxidant, which combines isolation and capture of superoxide radicals. Through TGA tests, higher temperature for thermal oxidation decomposition was found for TA-BP nanosheets, than those of pure BP nanosheets. Compared to pure BP nanosheets, exposing to ambient for 10 days, the lattice fringes and chemical bonds of TA-BP nanosheets weren’t destroyed, due to the hydrogen donors
Acknowledgements
The work was financially supported by National Natural Science Foundation of China (51761135113 and 51874266 and U1833113), Fundamental Research Funds for the Central Universities (WK2320000043).
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These authors contributed equally to this work.