Polyaniline-coupled multifunctional 2D metal oxide/hydroxide graphene nanohybrids.

Owing to their unique properties compared with conventional bulk analogues, two-dimensional (2D) nanomaterials, having nano-scale thickness and infinite length, are attracting increasing attention for their potential application in the fields of electronics, sensing, energy storage, and conversion. In particular, transition-metal chalcogenides and metal oxide nanosheets are highly promising functional 2D nanomaterials, however, their synthesis in a large scale remains a great challenge. Graphene, a 2D “aromatic” monolayer of carbon material having an ultralight weight, high surface area, and electric conductivity, has emerged as an ideal substrate for the growth and anchoring of functional nanomaterials, such as metal oxide/hydroxide nanoparticles (MO/MH NPs). 17–19] The strong coupling between MO/MH NPs and graphene in a confined 2D manner gives nanohybrids with unique structural features and synergistic physical and electrochemical properties derived from both counterparts. Along this line, numerous 2D functional nanohybrids comprising MO/MH NPs and graphene have been successfully constructed. Nevertheless, owing to the general incompatibility between graphene and inorganic NPs under synthetic conditions, the growth of MO/MH NPs on a graphene substrate with uniform morphology, controllable particle size, and enhanced coupling effects constitutes a highly desirable synthetic target. Polyaniline (PANI) is a typical low-cost conducting polymer that can be readily shaped into multiform morphologies, such as fibers/tubes, dots/shells, and other oriented nanostructures. Hydrothermal treatment has proven to be an excellent strategy for fabricating PANI NPs. Therefore, given that the protonated nitrogen atoms in PANI can bind with metal ions and mediate their hydrolysis process during hydrothermal treatment, we imagined that graphene-supported PANI nanosheets can facilitate the growth of MO and MH nanocrystals, and thus give rise to ternary nanohybrid sheets with a uniform distribution of hybrid NPs. Herein, we demonstrate an efficient and universal strategy for the controlled growth of MO/MH (such as Co3O4, Fe2O3, and Ni(OH)2) NPs on graphene to construct unique 2D nanohybrids employing PANI as the coupling linker between the two components. These nanohybrids have a welldefined 2D morphology, confined MO/MH NPs within the PANI nanostructures, controllable particle size, and high specific surface areas. The fabricated ternary hybrids of graphene, PANI, and Co3O4 (G-PANI-Co3O4) with a particle size of 6 to 10 nm deliver excellent rate capability and cycle performance when used as electrode materials for supercapacitors. Further, thermal treatment of G-PANI-MOs/MHs under inert gas yields nitrogen-doped carbon nanosheets integrated with size-controlled metal NPs (GNC-M). For example, N-doped carbon nanosheets supported by 3 to 5 nm sized cobalt NPs (GNC-Co) are synthesized by pyrolysis of GPANI-Co3O4. The GNC-Co nanohybrids exhibit outstanding catalytic behavior for the oxygen-reduction reaction (ORR). The overall synthetic strategy of 2D ternary graphene, PANI, and MO/MH (G-PANI-MOs/MHs)-coupled nanohybrids is illustrated in Figure 1a. First, graphene oxide (GO) nanosheets were functionalized with polyaniline by in situ polymerization of aniline in a GO suspension, which led to the formation of GO-based polyaniline (GO-PANI) nanosheets after centrifugation. Second, at pH 7 the GO-PANI nanosheets were mixed with the corresponding metal salts as the precursors of MO/MH followed by hydrothermal treatment. During this process, PANI particles formed on the graphene surface, while MO/MH NPs grew simultaneously and were confined within the PANI nanostructures. Specifically, G-PANI-Co3O4 (1:3), G-PANI-Co3O4 (1:10), and GPANI-Co3O4 (1:20) were fabricated using GO-PANI with different GO to aniline weight ratios of 1:3, 1:10, and 1:20 respectively, in the first step of the synthesis. By the similar means, graphene decorated with PANI-Fe2O3 hybrid NPs (GPANI-Fe2O3 (1:10)) and PANI-Ni(OH)2 hybrid NPs (GPANI-Ni(OH)2 (1:10)) were also synthesized. For comparison, GO-PANI nanosheets were directly treated under the same conditions with no addition of metal salts, the composites obtained were named G-PANI. The morphology and microstructures of as-prepared GOPANI, G-PANI, G-PANI-Co3O4 (1:3), G-PANI-Co3O4 (1:10), [*] S. Li, Dr. Y. Su School of Aeronautics and Astronautics Shanghai Jiao Tong University 800 Dongchuan Road, Shanghai 200240 (P.R. China) E-mail: yzsu@sjtu.edu. cn