Reversible visible/near-infrared light responsive thin films based on indium tin oxide nanocrystals and polymer

In this study, we design a novel thermo- and photo-responsive nanocomposite film prepared by depositing indium tin oxide nanocrystals via the coating of amphiphilic copolymer on polycaprolactone substrates (INCP). The INCP film shows reversible surface morphology change properties by changing temperature as well as turning ON/OFF NIR laser. Especially, as the temperature changes from 25 to 75 °C, the film could regulate light transmittance from 75 to 90% across the visible and near-infrared region (500–1,750 nm). In addition, the film also exhibits excellent recycle and thermal stability at different temperature. Our results reveal that reversible surface morphology change properties are caused by curvature adjustment of film, which is owing to the coupling effect between copolymer and PCL with different thermal expansion strains. Our results suggest a possible strategy for the preparation of smart responsive materials in the future, which provides a reference for the development of new energy-saving materials.

Scientific RepoRtS | (2020) 10:12808 | https://doi.org/10.1038/s41598-020-69110-y www.nature.com/scientificreports/ peak in the NIR region owing to their surface plasmon resonance (SPR) effect [23][24][25][26] . Although ITO-NC as an essential transparent conducting oxide have been applied in many fields, these ITO-NC coating polymer as smart materials remain a major chemical challenge [27][28][29][30][31][32][33] . On the other hand, the previous work have drawn attention to develop temperature responsive polymers in liquid condition. However, the solvent evaporation upon solar heating is a predictable problem that can shorten the cycle life of the irreversible systems or devices to limit their practice applications 34 . In the future, the application of temperature-responsive polymers in device field will also be restricted. Therefore, to develop innovative thermo-responsive materials in solid condition with stable property, while being a formidable challenge, is highly desirable. Previous studies have found that cross linked bilayer with Poly(N-isopropylacrylamide) (PNIPAM) containing polycaprolactone (PCL) exhibits reversible folding properties because of swelling and shrinking of PNIPAM. Although some studies have reported PNIPAM and PCL as smart copolymers, the development of the inorganic nanoparticles coated with PNIPAM and PCL has not been reported 35-37 . Self-folding and self-wrinkling are also a universal phenomenon in nature. For example, the flower of oxalis rubra can open at daytime and selffolds at night. To study the mechanism of self-folding and self-curling phenomenon of plant is very meaningful to design novel smart responsive materials for application in saving energy fields.
Inspired by the aforementioned concepts, herein, we firstly prepared a novel thermo-and photo-responsive nanocomposite film by depositing tin oxide nanocrystals via the coating of amphiphilic copolymer (PMAO-PNIPAM) on polycaprolactone (PCL) substrates by dip-coated method. The film exhibits reversible optical regulation in visible and near-infrared region below 100 °C. We also investigate reversible surface morphology change process of the film by turning ON/OFF near-infrared light irradiation. Therefore, our study provides a new approach to build a solid-state photo responsive device for smart material in visible and near-infrared region.

Results and discussion
Here we report the fabrication of thermo/photo-responsive ITO-NC@copolymer film. The film is synthesized by two-step method. In the first step, indium tin oxide nanocrystals (ITO-NC) and copolymer (PMAO-PNIPAM) can be synthesized following the previously reported method (see Experimental Section and Supplementary Information 1). The peaks of XRD patterns of ITO-NC can be indexed as the cubic In 2 O 3 , which is consistent with the values in the literature (JCPDS 06-0416) in Fig. S1c. TEM images of ITO-NC and ITO-NC@copolymer show that the nanocrystals are predominantly spherical in shape, which are nearly monodisperse and an average size of ~ 5-10 nm ( Fig. S1a-b). ITO-NC exhibits strong absorption peak in the Vis-NIR region, as shown in Fig. S1d. Subsequently the surface modification of the ITO-NC@copolymer is carried out by ITO-NC coated with a layer of amphiphilic polymer PMAO-PNIPAM. The copolymer forms micelles in water, which encapsulate the ITO-NC. After coating with PMAO-PNIPAM, the ITO-NC modified with oleylamine are no longer soluble in cyclohexane but are highly dispersed in water. Next, the ITO-NC@copolymer is deposited from ethanol solution on the top of the polycaprolactone (PCL) film. The film can be called ITO-NC@copolymer/PCL (INCP) for short. Figure 1 illustrates the process and concept of the nanostructured of film. To investigate the surface morphology of INCP film, Atomic force microscopy (AFM) is carried as shown in Fig. 2. The AFM image shows that the surface thickness and roughness of INCP film are ~ 130-200 nm and 2.5 nm, respectively, which are applicable for photo-response and visible light transmittance (Fig. 2). Our results indicate that ITO nanocrystals and polymers are efficiently assembled into an INCP film, and the nanocrystals are uniformly dispersed in the polymer matrix, resulting in a nanoscale roughness of the film surface.  Fig. 3. Optical microscopy images reveal that the film has the shape of umbrellas including many tubes of ~ 5 μm diameters at room temperature, closely to those in previous reports of PNIPAM/PCL bilayers. 11 Fig. 3 shows that the film is wrinkling at temperatures at 25 °C and becomes plane at 55 °C. Then, the INCP film can complete the reversible surface morphology change reversible changes of surface morphology process in 1 min. The bilayer film shows reversible surface morphology change properties in response to temperature which are probably contributed by the interaction between PCL and PNIPAM. The PNIPAM layer shows swelling and collapse with temperature change, while the PCL layer restricts swelling and collapse of PNIPAM layer. As a result, the umbrella-like INCP film show reversible surface morphology change  To assess the optical properties of the film, samples are characterized by UV-vis-NIR spectrophotometer measurement. The film exhibits optical absorption in visible and near infrared region at room temperature with a broad absorption peak at ~ 1,200 nm. (Fig. 4a) The ITO-NC exhibit highly absorption efficiency in visible and near infrared region, which could be harnessed to boost the optical modulating ability in a broad wavelength range. Actually, similar absorbance features also exist in previously reported and are assigned to inter band transitions of indium tin oxide 26 . The absorption and transmission spectra of the film are investigated in the visible and near infrared region according to the temperature change (Figs. 4 and 5). The results show that the film exhibits thermo-responsive Vis-NIR absorption and transmission spectra during an increase of the temperature. The absorption intensities of the hybrid INCP film decrease with the increasing temperature from 25 to 75 °C (Fig. 4a). By contrast, the absorption intensity of INCP remains nearly unchanged when the temperature increases from 25 to 75 °C (Fig. 4b). Furthermore, the ITO film exhibits a NIR transmittance reversible change of 20% (from 95 to 75%) at 1,000 nm. However, the transmittance intensity of ITO-NC@copolymer nearly remains unchanged when the temperature increases from 25 to 75 °C. Therefore, the transmission properties of ITO-NC@ copolymer are not temperature sensitive. Although PNIPAM on the surface of ITO-NC is thermal-responsive polymer, the conformational change of the copolymer without PCL is not significant enough to alter the optical absorption efficiency in the near infrared region.
In order to demonstrate the advantages of the ITO-NC@ copolymer/PCL film, it is tempting to evaluate the thermal-responsive performance of ITO-NC coated with polyethylene glycol (PEG), and then deposited on the PCL film (ITO-NC@PEG/PCL, INPP), as a comparison experiment. The INCP film is also found to exhibit qualitatively similar absorbance spectra with an intense NIR absorption peak. However, the absorbance spectra and intensity of INPP remain nearly unchanged before and after increasing temperature (Fig. 6a). Obviously, the transmission properties of the ITO-NC@PEG/PCL film are not temperature sensitive. Our results suggest that the conformational change of PMAO-PNIPAM is not significant enough to alter the film. In addition, the PEG has little effect on the temperature induced reversible surface morphology change of the film. Furthermore, to  www.nature.com/scientificreports/ study the recycle stability of INCP film with temperature variation, we tested the light transmittance of INCP film at λ max = 1,000 nm between 25 and 75 °C, respectively (Fig. 6b). The results demonstrate that film exhibits reversible durability after 100 heating-cooling cycles between 25 and 75 °C. The light transmittance of INCP film is also completely reversible between 70 and 90%. In order to adjust the reversible change of surface topography more efficiently and conveniently, the films are not irradiated or irradiated with near infrared (NIR) laser (1,064 nm, 0.8 W/cm 2 ) for 1 min. The optical microscopy images are used to directly observe the film before and after NIR laser irradiation. The film also exhibits reversible surface morphology change induced by infrared light, in similar with changing temperature, as shown in Fig. 7. The INCP film is in a wrinkled state at room temperature (Fig. 7a). Once the NIR laser is turned on, the film is completely spread on the glass substrate within 1 min (Fig. 7b). After removal of the NIR laser, the film recovers to its initial wrinkling state (Fig. 7c). The surface morphology change of fims is completely reversible for ON/OFF switching actuated by the NIR laser, in which the ITO-NC act as a nano-heater to raise the local temperature of the film via the light-heat conversion of ITO-NC. When NIR laser is applied, the result reveals that the temperature of ITO-NC increases by photothermal effect (Fig. 8). Compared to previously reported SWNT/PNIPAM, the film exhibits interested tube-shaped structure 38,39 . The newly provided nanocomposite film may open a door for the design and development of NIR-driven devices and machines, which would be useful in smart material.

Conclusions
In conclusion, we have successfully designed a novel ITO-NC@copolymer/PCL (INCP) film, which exhibits reversible thermo/photo-responsive properties by changing temperature as well as taking infrared light irradiation. Particularly, the film could reversibly regulate light transmittance in the visible and near-infrared region with the increasing and decreasing temperature from 25 to 75 °C. In addition, the film also exhibits the reversible surface morphology change behavior controlled by NIR laser exposure or nonexposure. The findings demonstrate that coupling effect of ITO-NC@coplymer and PCL with different thermal expansion strains leads to the curvature adjustment of film. The thermo and photo induced nanocomposite film, with their visible and near infrared sensitivity, which provides a reference for the design of smart responsive materials in the future.