Fully Reversible and Super‐Fast Photo‐Induced Morphological Transformation of Nanofilms for High‐Performance UV Detection and Light‐Driven Actuators

Abstract Flexible and highly ultraviolet (UV) sensitive materials garner considerable attention in wearable devices, adaptive sensors, and light‐driven actuators. Herein, a type of nanofilms with unprecedented fully reversible UV responsiveness are successfully constructed. Building upon this discovery, a new system for ultra‐fast, sensitive, and reliable UV detection is developed. The system operates by monitoring the displacement of photoinduced macroscopic motions of the nanofilms based composite membranes. The system exhibits exceptional responsiveness to UV light at 375 nm, achieving remarkable response and recovery times of < 0.3 s. Furthermore, it boasts a wide detection range from 2.85 µW cm−2 to 8.30 mW cm−2, along with robust durability. Qualitative UV sensing is accomplished by observing the shape changes of the composite membranes. Moreover, the composite membrane can serve as sunlight‐responsive actuators for artificial flowers and smart switches in practical scenarios. The photo‐induced motion is ascribed to the cis–trans isomerization of the acylhydrazone bonds, and the rapid and fully reversible shape transformation is supposed to be a synergistic result of the instability of the cis‐isomers acylhydrazone bonds and the rebounding property of the networked nanofilms. These findings present a novel strategy for both quantitative and qualitative UV detection.


Nanofilm characterization
Fourier transform infrared spectroscopy (FTIR, Bruker) and X-ray photoelectron spectroscopy (XPS, AXIS ULTRA from Kratos Analytical Ltd.) were utilized for the characterization of the chemical composition of the CPTH-TFPA nanofilms.The binding energies were calibrated by setting the C1s peak at 284.6 eV.The surface morphology of the nanofilms was analyzed using emission scanning electron microscopy (SEM, SU8020, Hitachi).The morphology and thickness of the nanofilms were determined by atomic force microscope (AFM, Dimension Icon Atomic force microscope (Bruker) in ScanAsyst mode).The Young's modulus (E) of the nanofilms was measured in fast force mapping mode by AFM (Cypher VRS, Oxford Instruments), employing an AFM cantilever (AC240TS-R3, Asylum Research Probes).The tip radius of the cantilever was obtained using a relative calibration method, with PSFILM-12M (E = 2.7 GPa, Bruker) utilized for the calibration.The estimated mean value of the tip radius was 7 nm.In all the samples investigated with Fast Force Mapping mode, a force of 50 nN was exerted during the measurements.The internal structure of the nanofilms was examined using a high-resolution transmission electron microscope (HETEM, Tecnai G2 F20) at 5 kV.The X-ray diffraction (XRD) patterns and contact angles of the nanofilms were measured using Bruker D8 Discover and a video-based contact angle measuring system (OCA20, Dataphysics), respectively.Lastly, a solid-state UV-vis absorption spectroscopy study was conducted using a UV-visible near-infrared spectrometer (Lambda 1050, PE Company).

Interfacial adhesion strength between CPTH-TFPA nanofilm and PET membrane
The interfacial adhesion strength between the nanofilm and PET membrane was investigated by wear resistance tests using Tap300DLC AFM cantilever in contact mode.The spring constant of the AFM cantilever is 36 N/m, and the tip radius is 14 nm.Contact pressure was obtained by dividing the applied force by the cantilever tip surface area (2πr 2 ).The applied forces were 400 and 800 nN, corresponding to contact pressures were 283 MPa and 566 MPa, respectively.The scan rate was 0.8 Hz, and the resulting wear region was then imaged by using AC mode.As shown in Figure S9, the worn area can be clearly noticed in the resulting images after the wear tests under the applied forces of 400 and 800 nN, and the worn material has accumulated at the edges of the worn areas.However, the indentation depth of the scanning region is in the range of 2 ~ 10 nm, that is much smaller that the thickness of nanofilm (~ 35 nm).This means that the nanofilm was not delaminated from the PET membrane under the high contact pressure of 566 MPa, indicating the interfacial adhesion strength between the nanofilm and the PET membrane is high.Moreover, there is no individual nanofilm layer peeled off under the lateral forces, indicating the adhesion force between the nanofilm is strong.This is likely attributed to the attraction forces including hydrogen bonds and van der Waals forces between the nanofilms.Note: As shown in Figure S4, we found that at low temperature, the two monomers cannot react to form a membrane.At too high temperatures, the reaction between the monomers is too fast, resulting in an uneven film with poor robustness.The 50% humidity was employed because we observed that at lower humidity, monomers cannot accumulate at the air-DMSO interface, leading to the absence of a distinct nanofilm formation.This is due to there are fewer hydrogen bonds formed between water molecules and monomers.On the other hand, increasing humidity facilitates the accumulation of monomers, but it also introduces water molecules as catalysts in the reaction.This causes the monomer reaction rate to become excessively fast, resulting in uneven films and inferior mechanical properties.

Figure S5 .
Figure S5.The photographs of the system for UV light detection and partially enlarged photographah while the system is working.

Figure S6 .
Figure S6.The contact angles of the two sides of CPTH-TFPA nanofilm.

Figure S10 .
Figure S10.Wear experiments conducted with an AFM tip on the CPTH-TFPA/PET sample cured with 400 nN and 800 nN applied force.

Figure S11 .
Figure S11.Thickness dependence of the UV-induced displacement of the fabricated CPTH-TFPA/PET composite membranes.

Figure S12 .
Figure S12.The CPTH-TFPA/PET composited membrane at different humidity.Note: The experiment was realized by filling the vial with the vapor of a saturated solution of different salts (LiCl, MgCl 2 , NaBr, CuCl) at 25 ℃.

Figure S14 .
Figure S14.Experimental data and fitting curves of relative displacement changes of the prepared CPTH-TFPA/PET composite membrane versus UV power density at 375 nm.

Figure S16 .
Figure S16.The UV light response of a CPTH-TFPA/PET membrane after a storage of about 6 months.

Figure S17 .
Figure S17.The response of CPTH-TOFB/PET composite membrane to 254 nm UV light at different illuminances.

Figure S18 .
Figure S18.The response of the CPTH-TOFB/PET composite membrane to 254 nm UV light during consecutive 60 UV on/off cyclic operation upon 5.2 mW cm -2 UV light.

Figure S19 .
Figure S19.The response of the CPTH-ETBA/PET composite membrane to 310 nm UV light at different illuminances.

Figure S20 .
Figure S20.The response of the CPTH-ETBA/PET composite membrane during consecutive 80 UV on/off cyclic operation upon 4.5 mW cm -2 UV light.

Figure S21 .
Figure S21.The photos of the designed letter 'J' formed by CPTH-TFPA/PI membrane under different UV-light power, where the scale bars in all the digital photos are 1 cm.

Figure S22 .
Figure S22.The photos of the designed letter 'J' formed by CPTH-TFPA/PI membrane under different UV-light power, where the scale bars in all the digital photos are 2 cm.

Figure S24 .
Figure S24.Optimized S 0 and S 1 structures of the E-isomer.

Figure S25 .
Figure S25.Optimized S 0 and S 1 structures of the Z-isomer.

Figure S26 .
Figure S26.Frontier molecular orbitals of the E-isomer.