2.1. Characterizations of Materials
The Ag/MXene@chitosan hydrogel was prepared by bulk MAX etching to produce MXene nanosheets, Ag/MXene composited sheets preparation and their incorporation into chitosan hydrogel, as depicted in Figure 1 a. In detail, MXene nanosheets (Figure S1a-b, Supporting Information) were obtained by etching out the aluminum layer of MAX bulk phase (Figure S2a-e, Supporting Information) with a LiF/HCl mixture solution as etchants. After Ag nanoparticles deposition, from the scanning electron microscope (SEM) image and corresponding energy-dispersive X-ray spectroscopy (EDS) elemental mappings of Ag0.2/MXene0.8, it could be observed that the layered structure of MXene maintained and Ag nanoparticles were evenly distributed on the surface of MXene nanosheets (Figure 1b-d). The transmission electron microscopy (TEM) image of the Ag0.2/MXene0.8 further demonstrated that Ag nanoparticles with a size of 3~12 nm were uniformly decorated on the Ti3C2Txlayers (Figure 1e). Meanwhile, the high-resolution TEM (HRTEM) images showed that the lattice fringe with an interplanar spacing of 0.236 nm corresponded to the (111) plane of Ag nanoparticles (Figure 1f). Phase structure of the produced Ag/MXene composited nanosheets with various mass ratios were validated by the X-ray diffraction (XRD) patterns (Figure 1g). It was clear that Ti3C2Tx had been successfully prepared, as evidenced by the distinctive peak at 2θ = 6.1°. The characteristic peaks at 2θ = 38.2°, 44.4°, 64.6°, 77.6°, and 81.8° of the Ag/MXene composites could be assigned to the (111), (200), (220), (311), and (222) planes of Ag (JCPDS no. 87-0720), respectively. To optimize the loading of Ag nanoparticles (NPs), we prepared a series of Ag/MXene nanocomposites with various mass ratios. It was worth noting that when the relative proportion of Ag NPs was higher than 20 wt.%, the 2D morphology of MXene was destroyed, which might be attributed to the oxidation effect from silver nitrate (Figure S3a, Supporting Information).[30] Thus Ag0.2/MXene0.8 composite nanosheets were selected for the fabrication of AM/Ch gel. The survey scan X-ray photoelectron spectroscopy (XPS) spectra of pure MXene and Ag0.2/MXene0.8 were shown in Figure 1h, in which characteristic peaks originating from Ti 2p, C 1s, O 1s, F 1s, and Cl 2p orbitals could be observed. The peak at 368.5 eV in the composites could be assigned to the Ag 3d5/2 orbital, supporting the successful loading of Ag nanoparticles on the Ti3C2Tx layers. As shown in the high-resolution Ti 2p spectra (Figure 1i), the peaks of Ti species on the surface of Ag0.2/MXene0.8 changed from a low valence state to a higher valence state (Ti2+, Ti3+ → Ti4+) after Ag decoration, which further corroborated that the introduced silver nitrate oxidized the highly reducing Ti ions on the surface and this was believed to benefit the stability of MXene sheets.[31] In the core-level Ag 3d doublet spectrum (Figure 1j), the characteristic peaks at 374.51 eV and 368.51 eV could be indexed to Ag 3d3/2 and Ag 3d5/2 orbitals, and the splitting between the two distinctive peaks was 6.0 eV, demonstrating the existence of metallic Ag NPs in the Ag0.2/MXene0.8.
AM/Ch gel was synthesized by cross-linking chitosan chains through hydrogen bonding with Ag0.2/MXene0.8 as additives. As shown in Figure S4a-c (Supporting Information), after cross-linking, the fluidity of the added solution disappeared. Figure 1k showed cross-sectional SEM images of freeze-dried AM/Ch gel, demonstrating the porous networking structure, which was believed to enhance the water transportation during the solar steam generation process. AM/Ch gel also showed adhesive features and high stability (Figure S5a-b, Supporting Information). Additionally, the shape and size of the AM/Ch gel could be readily manipulated by changing the shape of mould, and in our case, we fabricated a toy rabbit with white chitosan hydrogel (described as Ch gel) as body and black AM/Ch gel as eyes, indicating the high flexibility of the hydrogel for various application purposes (Figure 1l). The phase structure of hydrogel samples after being freeze-dried was analysed using XRD (Figure 1m). The peaks at 2θ = 6.1°, 38.20°, 44.40°, 64.60°, 77.60°, and 81.76° of frozen AM/Ch gel could be attributed to above-mentioned characteristic peaks of Ti3C2Tx and Ag nanoparticles. In addition, the peak intensity of chitosan significantly decreased after cross-linking, which could be attributed to the reduced crystallinity. All these texture property characterizations clearly indicated the successful fabrication of flexible AM/Ch gel.