2D polymer refers to a sheet-like monomolecular macromolecule consisting of laterally connected repeat units with end groups along all edges. Due to its unique structure, 2D polymer has the advanced characteristics of polymers, porous materials and two-dimensional materials, and has become a hotspot in the research of new materials. However, existing synthetic methods, such as air-liquid and liquid-liquid interfaces, have problems such as difficulty in controlling thickness, and resultant 2D polymer shows poor crystallinity and low electrical properties.
In this work, we show a new method that overcomes these problems to synthesize crystalline 2D polyaniline films with lateral size ca. 50 cm2 and varied thicknesses from 2.6 nm to 30 nm. The synthesis is achieved by combining the advantages of air-water interface (i.e. confined reaction environment) and surfactant monolayer (i.e., soft 2D template). The achieved 2D PANI exhibits anisotropic charge transport and a lateral conductivity up to 160 S cm−1 doped by hydrogen chloride (HCl). Moreover, the 2D PANI displays superior chemiresistive sensing toward ammonia (30 ppb), and volatile organic compounds (10 ppm).
Figure 1. Schematic illustration of the synthetic procedures for 2D polyaniline.
The topic was actually initiated more than three years ago when I started my Postdoc position in Prof. Feng’s group in Jan. 2016. My research topic is interfacial synthesis of large area 2D polyaniline, which is initiated by my colleague Shaohua’s excellent works in the synthesis of mesoporous conducting polymer nanosheets (Angew. Chem. 55, 12516, 2016; Adv. Mater. 28, 8365, 2016). After a discussion with Dr. Zhikun Zheng (our research group leader), the first idea came to my mind is to use the Langmuir–Blodgett trough (LB trough) method, which was extensively using in our group to synthesize different kinds of 2D polymers at that time (Angew. Chem. Int. Ed. 54, 12058-12063, 2015; Nat. Commun. 7, 13461, 2016). After several tries, unfortunately, I could only observe small flakes on water surface, and these flakes show fibrous morphology under microscopy. Then, I tried the ice-templating method reported by M. J. Park et al. in Angew. Chem. (54, 10497, 2015), which results in 2D polyaniline with excellent crystallinity, however, the film shows only µm-scale homogeneity and its thickness (ca. 30 nm) cannot be well controlled. These problems origins from the nature of chemical reactions on solid (or solid-liquid) surface, where monomers has low mobility and the film formation is not confined. Anyway, this method gives me an important hint that I need a soft 2D template (which ideally has strong interactions with aniline monomers) to achieve my synthesis.
Then, I went back to my colleague Shaohua’s work (Angew. Chem. 55, 12516, 2016), which uses surfactant bilayers combined with block co‐polymer micelles to synthesize mesoscale-ordered polyaniline nanosheets in water. Here, the surfactant supramolecular assembly is exactly the soft 2D template that I have been looking for. Because, the surfactant not only forms bilayer assembly in water, but also forms highly order monolayer on water surface. These excellent works finally bring me an idea that the surfactant monolayer at air-water interface could be able to achieve the synthesis of large area 2D polyaniline crystal. In the next few weeks, I tried many different surfactants found in our lab (e.g. the perfluorinated carboxylic acids used in Shaohua’s work, steric acid, sodium stearate and octadecylamine, etc.). And I was excited to see large area polyaniline films appeared on water surface with some of these surfactant monolayers, which is in clear contrast to pure water surface. But these films still show poor crystallinities and rough morphologies under optical microscopy.
In order to find better surfactants, I searched literatures and was surprise to find that the surfactant monolayer method has been widely used around 1990’s for the crystallization of inorganic crystals (Nature 334, 692, 1988; J. Am. Chem. Soc. 115, 8497, 1993) as well as biomineralization of amino acids (J. Am. Chem. Soc. 111, 1436, 1989). And a very recent paper published by X. D. Wang et al. used sodium oleyl sulfate monolayer at air-water interface to synthesize ZnO nanosheets (Nat. Commun. 7, 10444, 2016). After trying almost all of these reported surfactant monolayers, I finally found that sulfate ions headed surfactants affords 2D polyaniline films with excellent crystallinity and morphological homogeneity. And the first 2D polyaniline crystalline film with tens of centimetres lateral size was obtained in Apr. 2016 with sodium oleyl sulfate monolayer templating.
During my preparation of the manuscript about 2D polyaniline and in Aug. 2016, my colleague Kejun Liu came to me to discuss the possibility of using my method in his work to synthesize polyimide based 2D polymers. After this discussion, I transferred my experimental parameters and on-using surfactants to Kejun’s work. And very luckily and excitingly, in the first try, we could able to see many excellent polyimide 2D crystals on water surface with sodium oleyl sulfate monolayer. In contrast, there was no crystal when the surfactant monolayer was absent on water surface.
Figure 2. Structural characterization of 2D polyimide.
Here, I would like to skip 10000 words on the long-term struggle with manuscript submitting, rejecting and corrections in the past 2-3 years. Anyway, I am happy to see that all reviewers gave very positive comments on this methodology, and my paper was finally accepted by Nature Communications (10, 4225, 2019) and Kejun’s paper accepted by Nature Chemistry (2019, Doi:10.1038/s41557-019-0327-5). By far, this methodology has been widely used in Prof. Feng’s group in synthesis of different kinds of 2D polymers (e.g. 2D conducting polymers, 2D COFs and 2D MOFs). Therefore, many exciting and inspiring results from this methodology can be expected in the near future!