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.