2. Results and discussion
Figure 1a illustrates the fabrication process for the MXene/PDA
composite film. First, MXene
(Ti3C2Tx) nanosheets are
prepared by selectively etching an Al layer in the
Ti3Al2C2 MAX phase. A
large number of surface functional groups, such as -O, -OH, and –F, can
be generated on the MXene nanosheets, as demonstrated through Raman
characterizations (Figure S1).[21, 22]Subsequently, DA molecules are adsorbed and further polymerized in situ
into the PDA macromolecules on the MXene nanosheets. PDA can be used as
a bridge between the MXene sheets to form a unique 3D cross-linked
structure. In addition, the hydrogen bond between MXene nanosheets and
PDA molecules facilitates the formation of the organic-inorganic hybrid
membrane after vacuum filtration. The MXene nanosheets are stacked in a
lamellar stacking structure after vacuum filtration (Figure 1b). The
fabrication of the pressure sensor is illustrated in Figure 1c. The
MXene/PDA composite membrane is designed as a sandwich structure by
covering both sides with flexible Cu electrodes. The flexible film is
further encapsulated in PVC films for continuous detection. The pressure
sensor can be integrated with intelligent devices for commercial medical
applications such as monitoring and analyzing the heartbeats and pulse,
movements, and subtle signals, including those of sound and swallowing
(Figure 1d), to achieve early warning of physical abnormalities.
In Figure 2a, the cross-section image of MXene/PDA composite film shows
an ordered layered structure with many interlayer gaps, and the PDA
molecules are embedded between the interlaced MXene nanosheets. Top-view
SEM image in Figure 2b indicates that the MXene/PDA film surface was
uniform and dense with the PDA molecules. Pristine MXene films are also
shown for comparison (Figure S2a and 2b). The atomic force microscopy
images further confirmed the rough surface morphology and change in
thickness after DA polymerization (Figure S2c). Transmission electron
microscopy (TEM) is used to clarify the morphology and composition of
the MXene/PDA composites. As shown in Figure 2c, both the lamellar
structure of MXene and amorphous structure of PDA can be observed in the
TEM image, showing that PDA was uniformly composited on the surface of
MXene. The XRD patterns of MXene/PDA films of different ratios are shown
in Figure 2d. The diffraction peak at 2θ=6.4° is attributed to the (002)
plane of the MXene nanosheets. Compared with the XRD pattern of the
Ti3Al2C2 MAX phase
(Figure S3), there is no characteristic peak at 2θ = 39°, indicating
that the product obtained by etching has no impurity-phase Al. To prove
the successful compounding of PDA and MXene, the (002) peaks of the
original Ti3C2Tx MXene
and composite films are further compared. After polymerization, the
blue-shifted (002) diffraction peaks indicates an increase in the
nanosheet spacing. The increased layer spacing clearly evidences that
the PDA is inserted between the
Ti3C2Tx MXene
nanosheets.[23] With increase in the DA content,
the (002) diffraction peak increases, however, the (002) peak shows
red-shifted with further improvement in the composite ratio with the
ratio of 1:3. Furthermore, the composite film becomes fragile and it is
no longer suitable for flexible monitoring of physical activities. The
structures of the MXene, PDA, and MXene/PDA composites are studied by
Raman spectroscopy in Figure S1, the Raman peaks at 204, 390 and 627
cm−1 corresponded to the active groups such as -O (A1g
of Ti3C2O2), -F
(Eg of
Ti3C2F2), and-OH
(Eg of
Ti3C2(OH)2),[24] respectively. The Raman peak at 1585
cm-1 is assigned to C=C stretching benzenoid ring. The
Raman peaks of the PDA molecular at 1338, 1415 and 1565
cm-1 are ascribed to C=O catechol stretching
vibrations.[19] The characteristic Raman peaks for
both MXene and PDA prove that the MXene/PDA composite have been
successfully prepared. The FTIR spectra of MXene and MXene/PDA composite
film are shown in Figure 2e. For the spectrum of MXene, a prominent
absorption band at 3452 cm-1 is observed, which could
be attributed to the stretching vibration of
-OH.[13] The strong absorption band at 3431
cm-1 assigns to the strong hydrogen bonding
interaction between MXene and PDA molecules in composite
film.[20, 23] The chemical states of MXene and
MXene/PDA composite are characterized by XPS. As shown in Figure S5, the
XPS survey spectrum indicates that the MXene nanosheets is mainly
composed of Ti, C, O and F elements.[25, 26] No
related peaks of Al element are detected, indicating that the Al layer
in the raw material Ti3AlC2 has been
completely removed during the etching
process.[27]. The peak intensity of the MXene/PDA
composite corresponding to F element is weaker than that of MXene, which
could be attributed to the change of the surface after compounding
dopamine (Figure 2f).[28] On the other hand, the
introduction of dopamine increases the content of N, O and C elements.
The fine chemical state of MXene and MXene/PDA is further studied. As
shown in Figure 2f, it shows the high-resolution Ti 2p spectra of
MXene/PDA composite film, which spectrum could be deconvoluted into four
peaks at 461.3, 457.1, 455.9, and 455.0 eV, which corresponded to Ti-C
2p1/2, Ti3+, Ti2+,
and Ti-C 2p3/2, respectively.[20,
29] The C 1s spectrum for the MXene/PDA flexible film can be divided
into three peaks centered at 286.2 eV, 284.8 eV and 281.9 eV, and
correspond to the presence of C-O/C-N, C-C and C-Ti bonds,
respectively.[30] The Ti-C peak could be
attributed to the Ti element in the
Ti3C2Tx nanosheets,
which is a special feature of the MXene. The C-N bond comes from the
PDA. The high-resolution O 1s spectrum of MXene/PDA containes three
specific peaks at 532.3 eV, 531.0 eV and 529.7 eV, which are attributed
to C-O, C-Ti-(OH)x and Ti-O bonds,
respectively.[29] Compared with the O 1s spectrum
of MXene in Figure S4c, the ratio of TiO2 is determined
by 41.9% is approaching 22.9%. The strong reduction of dopamine
molecules would inhibit the oxidizable nature of MXene. The specific
peak at 685.2 eV is attributed to the F-Ti bonds. The absence of F-Al
peak indicates the successful removal of Al element in
Ti3C2Tx.[29,
31]
The influence of dopamine on the sensing behavior is studied at a
pressure of 0.93 kPa by adjusting the mass ratios of the MXene
nanosheets and dopamine to 1:0, 3:1, 1:1 and 1:3,as shown in Figure S5.
The pressure response increases with the increase in the ratio of MXene
to DA (1:0, 3:1, 1:1). With an increase in the DA content, the
interlayer spacing gradually increases, which produces high deformation
and high response under loading pressure. As the ratio of DA is
increased further (1:3), the corresponding response decreases under the
loading pressure. The above results can be attributed to the following
reasons: (i) regulation of the conductive path from PDA to MXene
nanosheets. Polymers are typically weak conductors of electricity. After
compositing with PDA, the increased number of conductive paths of the
MXene nanosheets resultes in lower initial current. The interlamellar
spacing of the MXene nanosheets decrease with increasing in applied
pressure. As shown in Figure3a and 3b, dopamine molecules are embedded
in the MXene nanosheets,which increase the distance between them; this
is consistent with the XRD pattern analysis. The increased contact area
and conductive paths between the MXene flakes help improve the current
conduction of the composite membrane after the application of pressure.
(ii) The unique spherical-like structure of PDA provides good
deformation performance. The DA molecules are polymerized on the surface
of the MXene nanosheets, and a porous structure is fabricated. These
porous networks extending into the soft substrate can adjust the
compressive deformation distribution in the MXene/PDA hybrid film. Thus,
such a layered porous network structure can induce good
pressure-sensitive properties in flexible sensors. The I-V curves of the
flexible pressure sensor with the ratio of MXene to dopamine of 1:1 are
obtained at different voltages under different applied pressures. As
shown in Figure 3c, the current signal increases with the increasing of
the voltage, and the slope of the I-V curves is positively correlated
with the exerted pressure, indicating a stable sensitive performance.
The dynamic sensitivity curve of the MXene/PDA-based flexible sensor is
obtained under different pressure gradients. The sensitivity S is
defined as S= δ(ΔI/I0)/δP, where ΔI is
the difference between the current after the load pressure and the
initial current, I0 is the initial current, and P is the
pressure applied on the sensor.[23] As shown in
Figure 3d, the sensitivity of the device gradually increases in the
range of 0.18-6.20 kPa, and each dynamic response curve is uninterrupted
and stable. Figure 3e shows that the sensor could clearly distinguish
the signals generated by the water drops with different weight. As shown
in Figure 3f, the MXene/PDA-based flexible sensor exhibits fast
response/recovery speeds of 100 and 50 ms, respectively, and the fast
response and recovery capabilities ensure timely and flexible sensing
under the action of external forces. The relationship between
sensitivity and applied pressure in Figure 3g can be divided into two
areas: (i) in the low-pressure (0.18-2.90 kPa) range, the sensitivity of
S1 was 24.7 kPa-1; (ⅱ) in
high-pressure (2.90-6.20 kPa) range, the sensitivity of
S2 was 138.8 kPa-1. Overall, the
sensitivity of the MXene/PDA-based pressure sensor is higher than that
of the MXene film (Figure S6). The repeatability of the MXene/PDA
thin-film flexible sensor shown in Figure 3h indicates that the
sensitivity of the device is maintained after 350 cycles of compression
loading/unloading tests. The inset of Figure 3h shows that the
sensitivity of the pressure sensor is maintained at almost the same
amplitude over multiple compression cycles. As shown in Figure S7, the
morphology of the MXene/PDA composite film remained uniform and dense
after more than 350 loading/unloading cycles. Figure 3i compares the
sensitivity obtained in this study with that reported in the
literature.[14, 32, 33, 37-42] Compared with other
MXene-based flexible sensors, MXene/PDA-based pressure sensors generally
exhibit higher sensitivity, such as MXene/PVB-based porous composite
sensor (S1=11.9 kPa-1,
S2=1.15 kPa-1, S3=0.20
kPa-1),
Ti3C2Tx/PVDF-TrFE-based composite
nanofiber sensor (0.51 kPa-1), elastic
microstructure-based MXene/PDMS thin film sensor (S1=2
kPa-1, S2=0.003
kPa-1) MXene@CS@PU-based pressure sensor (3
kPa-1), pressure sensor based on MXene/Cotton (12.095
kPa-1), pressure sensor based on MXene/PI (22.32
kPa-1), thin-film sensor based on LS-MXene/PVA
(S1=5.5 kPa-1, S2=1.5
kPa-1), elastic aerogels of MXene/rGO (22.56
kPa-1) and pure MXene-based thin film sensor. In
addition, the response and reply times of our proposed sensor are
shorter than those mentioned above, enabling higher-accuracy detection.
As dispalyed in Figure 4a, the MXene/PDA flexible pressure sensor with
the flexibility and biocompatibility can be integrated with intelligent
system for health monitor, owing to its high sensitivity over a wide
sensing range, which can be used for health warning, physiological
monitor, dignose deasease and telemedicine ranging from small
physiological signals to human posture. Figure 4b shows the
MXene/PDA-based composite thin-film flexible pressure sensor attached to
the curved skin of the index finger joint to stably record the real-time
signal changes caused by different finger motions of varying amplitudes,
thereby distinguishing between the different bending angles of the
fingers (from 30° to 90°). The pulse of the human wrist has clinical
significance for indicating the changes in the heart rate and arterial
conditions. Therefore, the use of pressure sensors to detect the pulse
on a person’s wrist has immense value.. As shown in Figure 4c, the
MXene/PDA flexible pressure sensor can be attached to the human wrist to
accurately detect the pulse and diagnose potential diseases. The
enlarged waveform of the heartbeat with two distinguishable
characteristic peaks is displayed, which can be ascribed to the systolic
(P1) and diastolic peaks
(P2).[33] These results further
confirm that the MXene/PDA flexible sensor has excellent sensitivity
even when detecting small strains. When different words are spoken, the
strain sensor shows different characteristic peaks, that is, different
syllables are reflected as changes in the sensor current. The words
”flexible” and ”carbon” may have complex throat movements, which
generate different peak-shaped patterns and differences in insensitivity
in Figure 4d. Excellent voice recognition capabilities make the sensor
very promising for voice recovery exercises and human–computer
interactions. It holds great promise in helping speech-impaired people
convey information by recognizing vocal signals. In addition, this
flexible sensor can recognize subtle changes in the facial expression.
For example, when the eyebrows are raised, there was an obvious response
with a △I/I0 value of approximately 12 (Figure 4e).
Furthmore, it can sensitively identify the bending of devices at
different angles; the bending angles of 30°, 45°, 60°, and 90° generate
obvious current signal changes, where the bending angle is positively
correlated with the sensitivity (Figure S8). Furthermore, a touch switch
system is assembled to realize light adjustment using pressure, which
has potential applications in intelligent control. A circuit diagram of
the MXene/PDA-based pressure sensor is shown in Figure 4f. The LED is
dark in the absence of external pressure but lighted up faintly after
placing a small screw. The brightness of the LED light gradually
increases with the increasing of the applied pressure, indicating the
decrease in resistance in the flexible device. Moreover, these results
further indicate that flexible devices based on MXene/PDA films may have
broad prospects for the development of intelligent controllers and
switches, which have applications in various intelligent fields, such as
human-computer interaction and intelligent control.