Figure 1 Step by step preparation of AO-Fc adsorbent material
The chemical structure was confirmed by FTIR and XPS analysis. The FTIR spectra of Fc, CN-Fc, CN-Fc1 and AO-Fc were shown in Figure 2a. All spectra have similar shapes in the vibration band of carbonaceous material and a peak at 3500-3700 cm-1 was observed due to the presence of OH groups36,37. Afterward, Fc was modified with the aid of azobisisobutyronitrile and divinylbenzene to yield CN-Fc. On comparing the FTIR spectrum of CN-Fc and CN-Fc1, the peaks in the spectra at 2242 cm-1 were assigned to C≡N and high peak intensity witnessed the higher payload of nitrile groups were seen on CN-Fc than CN-Fc138. The post-synthetic functionalization further converted C≡N groups into C=N, C-N and N-O confirmed by respective peaks at 1638 cm-1, 1380 cm-1 and 928 cm-1 respectively, indicating the presence of amidoxime groups on the surface of Fc39. The presence of amidoxime groups were further investigated by XPS analysis (Figure 2b-d); in C 1s high-resolution spectra of AO-Fc (Figure 2b) showed two peaks at 284.45 eV and 285.5 eV assigned to C-N and C=N; in N 1s spectrum of AO-Fc (Figure 2c) showed two peaks at 400.05 eV and 398.40 eV assigned to N-H and C=N respectively36, and the O 1s spectra of AO-Fc (Figure 2d) showed two peaks at 532.5 eV and 531.9 eV assigned to C(NH2)-N-OH and O-H groups respectively40. The powder XRD patterns of Fc, CN-Fc and AO-Fc were shown in Figure 2e. The XRD patterns showed peaks around 16.2o which can be indexed (101) plane and confirmed the amorphous nature of the material41. Furthermore, a broad peak was seen at around 25o and a small peak nearly 43o, which can be indexed (0002), (1010/1011) plane of graphite and are consistent with the presence of amorphous carbon42. The contribution of amidoxime groups on the surface of AO-Fc was calculated by carrying thermogravimetric analysis. Compared with Fc, a significant weight loss (80 %) was observed in AO-Fc in the temperature range of 300 oC to 550oC (Figure 2f). The higher weight loss in AO-Fc was due to the large amount organic groups were thermally decayed, which confirmed the weight contribution of amidoxime groups taken to 80 % of the weight loss of AO-Fc.
Figure 2 FTIR spectra (a), high-resolution XPS spectra of C 1s (b) N 1s (c) and O 1s (d), powder XRD pattern (e), and TGA curves (f)
SEM morphology of the vertical and horizontal cross-section of Fc, CN-Fc, CN-Fc1, and AO-Fc were shown in Figure 3. The structure of Fc was seen with two-dimensional rectangular two open-end vessels arranged in a proper array with a breadth of 28.6 μm (Figure S2a) and height of 18.3 μm (Figure 3abc) and (Figure S2b). SEM images of CN-Fc and CN-Fc1 were not changed obviously after chemical modification (Figure 3de). However, high magnified SEM images showed the rectangular walls were covered with small structures that confirmed the presence of chemical groups. Comparison to Fc, AO-Fc SEM images showed thick wall rectangular vessels (high magnified image) (Figure 3f) without any collapse or damage with a height and breadth of approximately 27.02 μm (Figure S2c) and 14.75 μm (Figure S2d). In other words, the AO-Fc was successfully prepared by amidoximation reaction of nitrile groups.