4. Materials and Methods
Materials. 3-Hydroxytyramine hydrochloride (dopamine) was
purchased from Adamas. Lithium fluoride (LiF, 99.9%) was purchased from
Adamas. Ti3AlC2 MAX phase (400 mesh) was
purchased from 11 Technology Co., Ltd. Hydrochloric acid (HCl, 37%) was
purchased from Adamas.
Synthesis of MXene nanosheets . To synthesize
Ti3C2Tx nanosheets from
Ti3AlC2 MAX phase, the etching solution
is LiF/HCl.[55] First, a certain amount of LiF and
HCl (9 M) was stirred for 30 min to ensure that LiF is dissolved. Then
the solution was heated to 35° C and the precursor MAX phase was slowly
etched at a certain speed for 24 h to etch the Al layer. The resulting
solution was repeatedly washed with deionized water until the pH value
was higher than 6 to obtain a few layers of
Ti3C2Tx nanosheets.
MXene/PDA biocomposite film preparation. First of all, the
concentration of MXene solution was calculated by vacuum filtration.
Subsequently, dopamine of different masses was used to control the mass
ratios to 1:0; 1:1/3; 1:1 and 1:3. We stirred the solution magnetically
for 30 minutes at a certain speed, then adjusted the pH of the solution
to about 8.5, and continued to stir continuously for 12 hours under
certain conditions. Subsequently, the solvent was removed and a
composite membrane was formed by vacuum filtration using a cellulose
acetate membrane as a substrate. At last, it was named as MXene/PDA film
and dried overnight at room temperature.
Fabrication of flexible pressure sensor. Copper films were first
drawn from both ends of the MXene/PDA flexible film as electrodes, and
then the two flexible PVC films were cleaned with deionized water,
ethanol and acetone, respectively, and covered at both ends of the
pressure-sensitive material to prevent material contamination and
ensured the stability of the equipment during the test.
Characterization . The crystalline structures of MXene film and
MXene/PDA were characterized by X-ray diffraction (XRD, D8-Advance,
Germany) with 2θ in the ranges of 5~60° at room
temperature. The morphology of the
Ti3C2Tx MXene film and
MXene/PDA biocomposite film were observed with field emission scanning
electron microscopy (FE-SEM, JEOL JSM-7001F, Japan) and transmission
electron microscopy (TEM, Tecnai G220S-Twin, USA). The thickness and
surface topography of sensitive materials were characterized by atomic
force microscopy (AFM, Cypher, USA). The surface compositions and
chemical states were examined by X-ray photoelectron spectroscopy (XPS,
VG ESCALAB 210, USA). The sensing performances are measured by the
flexible device analysis system (AES-4SD, SINO AGGTECH, China).