Ternary nanocomposites of reduced graphene oxide, polyaniline and hexaniobate: hierarchical architecture and high polaron formation

Nanostructured systems, such as nanocomposites, are potential materials for usage in different fields since synergistic effects of their components at the nanoscale domain may improve physical/chemical properties when compared to individual phases. We report here the preparation and characterisation of a new nanocomposite composed of polyaniline (PANI), reduced graphene oxide (rGO) and hexaniobate (hexNb) nanoscrolls. Atomic force microscopy images show an interesting architecture of rGO flakes coated with PANI and decorated by hexNb. Such features are attributed to the high stability of the rGO flakes prepared at room temperature. Detailed characterisation by X-ray photoelectron and Raman spectroscopies indicates an intermediate reduction degree for the rGO component and high doping degree of the PANI chains compared to the neat polymer. The latter feature can be attributed to cooperative effects of PANI chains with rGO flakes and hexNb nanoscrolls, which promote conformational changes of the polymer backbone (secondary doping). Spectroscopic and electrochemistry data indicate a synergetic effect on the ternary nanocomposite, which is attributed to interactions between the components resulting from the morphological aspects. Therefore, the new nanocomposite presents promising properties for development of new materials in the film form on substrates for sensing or corrosion protection for example.

3 h in order to dissolve NaNO3 pellets. The suspension was cooled with ice/water bath and 45 g of KMnO4 (Sigma-Aldrich) was very slowly added (over 1.5 h). Ice/water bath was then removed and the mixture was stirred for 7 days. The viscous gel-like dispersion of graphite oxide obtained after purification process by washing/centrifugation with dilute H2SO4 and H2SO4/H2O2 solutions, followed by washing/centrifugation with deionized water for several times, as described by Rourke and co-workers [2]. The resulting dispersion was diluted with deionized water and high-shear mixed at 7000 rpm for four times (15 min each). GO particles prepared by this method present 100% of monolayer content and flake sizes ranging from 5 to 30 μm [3,4], which are remarkably larger in comparison to GO reported in literature obtained by sonication (less than 10 μm) [2,[5][6][7][8][9][10].
Reduced graphene oxide (rGO) samples were prepared under different conditions and evaluated for the preparation of the hybrid materials. Different experimental parameters were explored, such as GO concentration (from 0.12 to 0.25 mg/mL), temperature (25 and 80 °C) and time (from 3 h to 7 days). Such parameters are very important both for the stability of the dispersions and the degree of oxidation/reduction of GO to rGO [5][6][7][10][11][12]. For the preparation of the nanocomposites describe in this paper, GO stock dispersion was diluted to 0.25 mg/mL, and 90 mL of such dispersion was collected in a 120 mL flask and kept at 25 °C.
Then, 400 μL of ammonia solution (Sigma-Aldrich, 25%) were added and the dispersion was stirred for 20 min. Diluted hydrazine hydrate solution (Sigma-Aldrich, 8%, 273 μL) was S3 slowly added and the mixture was kept under stirring at 25 °C for 7 days. The pH value of the resulting rGO dispersion was 8.7.

Preparation of dispersion of emeraldine salt polyaniline (PANI-ES)
Dispersions of PANI-ES in water/N,N-dimethylacetamide (pH 2.5) was prepared following procedure from literature [13,14] to prepare the nanocomposites, as described in the paper.

Exfoliation of H2K2Nb6O17
The exfoliation of protonic hexaniobate was performed by following procedure described by Shiguihara and co-workers [16,17]

Preparation of PANI/hexNb reference sample
A polyaniline/hexaniobate reference sample was prepared by mixing PANI and hexNb dispersions (concentrations of 2.8 mg/mL and 1.12 mg/mL, respectively). The volumes of the dispersions were such that PANI/ hexNb weight ratio was ca. 0.50 (same value as for the ternary nanocomposite). Figure 1 shows the optical images of the dispersions of rGO prepared by reducing GO with hydrazine at room temperature for 7 days (rGO-25) and the corresponding rGO/PANI nanocomposite dispersion, before and after resting for 5 h. The images in Figure 1 show that rGO-25 and rGO/PANI nanocomposite are stable in dispersion, although reduced graphene oxide samples are well known to present high aggregation in aqueous media.

S5
As discussed in the paper, XPS spectra at the C 1s and N 1s core levels were fitted in order to provide detailed structural information on the nanocomposite and graphene oxide samples.
Peak fitting for such high resolution spectra was performed by applying tight constrains for binding energy range, FWHM and shape of components, based on a comprehensive assessment of the literature [2,7,9,11,12,[18][19][20]. Table 1 and Table 2 show the fitting data and results for the C 1s and N 1s core level spectra, respectively. S6 Table 1: Fitting data and results of high resolutions XPS spectra at C1s core level of graphene oxide (GO) and reduced graphene oxide (rGO-25 and rGO-80) samples.