Room-temperature Synthesis of Inorganic-organic Hybrid Coated VO2

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Applications of Polymer, Composite, and Coating Materials

Room-temperature Synthesis of Inorganic-organic Hybrid Coated VO2 nanoparticles for Enhanced Durability and Flexible Temperature-responsive Near-infrared Modulator Application Shuwen Zhao, Ying Tao, Yunxiang Chen, Yijie Zhou, Rong Li, Lingling Xie, Aibin Huang, Ping Jin, and Shidong Ji ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19881 • Publication Date (Web): 18 Feb 2019 Downloaded from http://pubs.acs.org on February 20, 2019

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ACS Applied Materials & Interfaces

Room-temperature Synthesis of Inorganic-organic Hybrid Coated VO2 nanoparticles for Enhanced Durability and Flexible Temperature-responsive Near-infrared Modulator Application Shuwen Zhao†,‡, Ying Tao†,§, Yunxiang Chen†,‡, Yijie Zhou†,‡, Rong Li†,‡, Lingling Xie†,ǁ, Aibin Huang*,†, Ping Jin†,§ and Shidong Ji*,† †State

Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai

Institute of Ceramics, Chinese Academy of Sciences, Dingxi 1295, Changning, Shanghai, 200050, China ‡University

§School

ǁInner

of Chinese Academy of Sciences, Beijing 100049, China

of materials science and Engineering, Shanghai University, Shanghai 200444, China

Mongolia Normal University, Inner Mongolia 010022, China

KEYWORDS: flexible sensors, durability/stability, vanadium dioxide (VO2), inorganic-organic hybrid, magnesium fluoride (MgF2), infrared modulator

ACS Paragon Plus Environment

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ABSTRACT: Vanadium dioxide is one kind of desirable infrared modulators for sensors because of its remarkable temperature-responsive infrared modulation ability via autogeneic metalinsulator transition. However, the detriments of poor chemical stability and narrow scope of extensive-researched application (e.g. smart windows) restrict its mass production. Here, we propose a VO2@MgF2@PDA inorganic-organic hybrid coated architecture for great enhancing the optical durability more than 13 times in contrast to the pristine VO2, meanwhile the transmittance difference between room and high temperature changed within 20% (decreasing from 25% to 20.1%) at λ = 1200 nm after the ageing time of 1000 h in constant temperature (60 °C) and relative humidity (90%). Further, based on the as-synthesized durability-enhanced nanoparticles, we fabricated a flexible sensor for temperature-field fluorescence imaging by integrating the VO2-based near-infrared modulator with the upconversion fluorescence material. Additionally, the formation mechanism of VO2@MgF2 core-shell nanoparticles was studied in details. The inorganic-organic combination strategy paves a new way to improve the stability of nanoparticles, and the VO2-based flexible temperature-fluorescence sensor is a promising technique for remote and swift temperature-field distribution imaging on the complicated and campulitropal surfaces.

1. INTRODUCTION Flexible sensors, a kind of sensors that is compatible with movable and arbitrary parts or campulitropal surfaces, are expected to apply to a new paradigm in large-area electronics. By contrast with conventional sensors, flexible sensors can capture objective analytes more effectively and produce higher-quality signals avoiding the rigidity of analytes and the poorquality signal transduction. Recently, rapid developments in the design and fabrication of


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flexible sensors lay the foundation for marked expanding the application field of sensors like the aforesaid curved geometries and collapsible systems.1-2 Various kinds of application paradigm of flexible sensors, such as flexible ultraviolet sensors, organic field-effect transistors, flexible pH sensors and ion sensors,3 have been reported. Among these flexible sensors, photonic-based sensors possess the characteristics of high precision, fast response and non-contact detection, which plays very important role in our daily life. Recently reported flexible light sensors focus mainly on ultraviolet region (300–400 nm), however, the much wider wavelength range at infrared (IR) (>780 nm) spectra have not gotten enough attention. IR light has the outstanding ability that transmit through atmosphere, biological tissue, and other materials,4-5 and different IR spectral range can be used for biological imaging (800–1100 nm), telecommunications (1300–1600 nm), thermal imaging (>1500 nm), thermal photovoltaics (>1900 nm), and solar cells (800–2000 nm),6 suggesting that the flexible IR sensors have great potential for medical, industrial, communication, nightvision, military territory, and security applications. Hence, controlling and manipulating the spectral IR range is an inevitable requirement that provides spectral selectivity for practical applications. Monoclinic vanadium dioxide (VO2) exhibits an ultrafast (picoseconds timescale) first-order metal-insulator transition (MIT) at ~340 K,7-8 accompanied by an abrupt regulation of whole IR spectral region. This performance shows a remarkable temperature-response IR modulation characteristic which can be used as modulator in IR sensors. However, common method of VO2 films producing is to deposit on a rigid substrate through reactive sputtering process and wetchemical route.9 Usually, the sputtering process needs high temperature (>450 °C) or post-heat treatment, resulting in especially difficult and complicated preparation of flexible VO2 films. In addition, the as-prepared optical film products have large regulation located at the wavelength


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beyond 1500 nm,10-11 which hinders the modulation ability of VO2 films in IR range of 10001500 nm. According to previous reports,9, 12 the wet-chemical route is a facile, low-cost, largescale-produced, and versatile approach, meanwhile the as-prepared films have the remarkable regulation ability in 1000-1500 nm arouse from the localized surface plasmon resonances (LSPR) of VO2 nanoparticles.13 Unfortunately, it has been revealed that these nanoparticles are liable to react with oxygen, moisture, and resin,14-16 resulting in losing its IR modulation properties. Very recently, a lot of efforts have been made to improve the environmental durability of VO2 films, and one effective method is to coat VO2 nanoparticles with an inert barrier shell (i.e. core-shell structure) to isolate from H2O, O2, and active groups of polymer. Many oxides, such as zinc oxide,15 alumina,16 titania,17 and silica,14 have been covered on the surfaces of VO2 nanoparticles for protection. Nevertheless, most of these inert shells alone (such as alumina, titania, silica) are still not enough to meet the durability requirement of practical application. The major reason is that the inorganic shell is continuous but uncompact as some cracks or pores exist in the shells. The formation of these bulk defects could be caused by the volume change (~0.12%) during the MIT transition and the self-generation at the core-shell synthesis process. Thus, the fabrication of long-term stable VO2 nanoparticles and the corresponding films for industrial applications is still a challenge, which needs to be overcome as soon as possible. Organic polymers show great potential in adhering to the outside of the shell and encapsulating the bulk defects, which can insulate the damage compositions efficiently. Hence, inorganicorganic hybrid core-shell structure could provide a promising strategy for better environmental durability of VO2 films. Here, we selected magnesium fluoride (MgF2) as the inorganic shell component because of the same tetragonal crystal system (space group P42/mnm, unit lattice


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parameters: a=4.62 Å and c=3.05Å) with rutile VO2 (P42/mnm, a= 4.55 Å, c= 2.86 Å) (Figure 1a). At the same time, it was reported that MgF2 can be controlled for the preparation of hollow particles of different shell structure and cavity diameter with excellent stability (e.g., corrosion resistance, thermal stability, photo-stability) and properties (e.g., low refractive index, significant hardness, transparent visible-IR region)18, implying that MgF2 is a good candidate as an inert shell for VO2 nanoparticles protection. Moreover, polydopamine (PDA) was selected as organic adhesive since it can form robust and strong adhesiveness to nearly any surfaces, which is beneficial for stuffing the cracks of inorganic shell layer.19 In this work, the reaction parameters of VO2@MgF2 preparation, including solvent, temperature, the molar ratio of Mg:F and VO2:MgF2, and the injection rate of NH4F components, were discussed in details. The environmental durability of pure inorganic MgF2 shell (VO2@MgF2) can enhance by ~6.5 times than pristine VO2 film. While after the coating of PDA on VO2@MgF2, the durability of VO2@MgF2@PDA can further increase to more than 13 times. The durability-enhanced nanoparticles were further fabricated into flexible near-IR modulator through combining with the upconversion fluorescence material for temperature-field fluorescence imaging. This inorganicorganic combined coating strategy opens up a new avenue for improving the stability of nanoparticles through selectively passivating their reactive edges and promotes the application of flexible VO2 films in flexible sensors and telecommunications fields. 2. EXPERIMENTAL SECTION Preparation of VO2@MgF2 Core-shell Structure Nanoparticles. The 3 mmol (0.25 g) of VO2(M) nanoparticles (prepared following our previous work20) were ultrasonically dispersed in 30 g of solvent, including solvent of deionized water and ethylene glycol (EG, Shanghai Titan Scientific Co., Ltd.), to form a well-dispersed suspension. Then, appropriate (VO2:MgF2 = 0.2,


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0.5, 0.8, 1, and 1.5) magnesium chloride hexahydrate (MgCl2·6H2O, Aladdin Reagent Co., Ltd.) was dissolved in 10 mL of solvent and added into the suspension. The suspension was heated to different temperature (20, 40, 60, 80 °C) and maintained 1 h. Afterward, the 10 mL of solvent dissolved with corresponding amount (Mg:F = 1:0.25, 1:0.5, 1:1, 1:2) of ammonium fluoride (NH4F, Aladdin Reagent Co., Ltd.) were injected into the above suspension at a certain rate under vigorous stirring. After 30 min, the suspension was centrifuged and the final VO2@MgF2 nanoparticles were washed with deionized water and ethanol (Sinopharm Chemical Reagent Co., Ltd.) at least 3 times, and then disperse in ethanol for further use. Preparation of VO2@MgF2@PDA Inorganic-organic Core-shell Nanoparticles. The VO2@MgF2@PDA was prepared by means of a method described in previous reports.21-22 Dopamine powder (Aladdin Reagent Co., Ltd.) was dissolved in Tris solution (pH 8.5, 10 mM Tris-HCl buffer, Aladdin Reagent Co., Ltd.) with a concentration of 2 g/L. The 0.1 g of VO2@MgF2 was dispersed in 40 mL above Tris solution and allowed to wait for 12 h under stirring at room temperature. The resultant product was collected by centrifugation and washed three times with water. Preparation of Temperature-responsive Flexible Near-infrared Modulator Film. The 0.15 g of as-prepared products were dispersed ultrasonically into 5 g of ethanol to form uniform suspension, whereas 5 g of polyvinyl butyral (PVB, M.W. 90 000-120 000, Aladdin Reagent Co., Ltd.) was added into the suspension and stirred for 10 min. The slurry was uniformly cast on the poly(ethylene terephthalate) (PET) substrate and dried at 70 °C for 1 h. The thicknesses of flexible films can be easily controlled by scraper size. The whole process for the preparation of flexible films is illustrated in Scheme 1.


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Scheme 1. Experimental flow chart for the whole process of flexible films preparation. Characterization. The X-ray diffraction (XRD, Model D/Max 2550 V, Rigaku, Japan, Cu Kα radiation source λ=1.5418) was applied to identify the crystal phases of the samples at the voltage of 40 kV and the current of 40 mA. The microscopic morphology of the samples was characterized by using a field emission scanning electron microscope (SEM, Magellan 400, FEI, America) and a high-resolution transmission electron microscope (TEM/HRTEM, JEOL, JEM2010). The organic core-shell nanoparticles were determined by fourier transform infrared spectroscopy (FTIR) with Thermo Scientific Nicolet iS 10, and the scanning range of the wavenumber was 400-4000 cm−1. The durability and near-IR modulator properties of the flexible films were measured using a UV-Vis-near-IR spectrophotometer (HITACHI U-3010) equipped with a temperature control accessory. The samples were placed in damp heating environment (T=60 °C, RH=90%) for several days and then their relevant properties were tested at predetermined time intervals. The phase transition characteristics of VO2 were detected by


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differential scanning calorimetry (DSC200F3 NETZSCH) at the temperature range of 20-80 °C under the heating/cooling rate of 10 °C/min. 3. RESULTS AND DISCUSSION Determination of reaction parameters of VO2@MgF2 synthesis. A series of VO2@MgF2 nanoparticles were synthesized by adjusting the reaction parameters including the reaction solvent, temperature, the molar ratio of Mg:F and VO2:MgF2, the injection rate of NH4F components. First, deionized water was selected as the solvent, as shown in Figure S1, the phase structure of VO2 nanoparticles have not changed after the preparation process of core-shell structure. However, it is worth mentioning that the XRD intensity of the samples decreased with the increase of the temperature, which could be ascribed to the poor chemical stability of VO2. It can be dissolved and destroyed by the solvent at a higher temperature, leading to the weaker crystallinity of VO2 nanoparticles with the rise of reaction temperature. The photograph of reacted supernatant (Figure S2) also supports this conclusion. Second, the apt ratio of VO2:MgF2 at 20 °C in deionized water was determined by TEM characterization. Figure S3 shows the TEM results of the different molar proportions of VO2:MgF2 including 1:0.2, 1:0.5, 1:0.8, and 1:1. It is evident that the molar ratio of VO2:MgF2 = 1:0.2 has inadequate MgF2 components to form coreshell structure but floccules, on the other hand, the ratio of VO2:MgF2 = 1:1 has excessive MgF2 components to form nanoparticles not core-shell structure. The appropriate molar ratio of VO2:MgF2 is 1:0.8, which can prepare uniform core-shell nanoparticles (thickness 10~20 nm). Third, the different molar ratios of Mg:F for VO2@MgF2 synthesis was analyzed by XRD and TEM as shown in Figure S4 and Figure S5, respectively. Figure S4 reveals that the change of Mg:F has not had great effects on the diffraction peaks of VO2, but the XRD intensity of a peak in the circle which has been proved to relate with the diffraction of MgF2 in Figure 1b and c


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increases with the reduce of Mg:F. This phenomenon well coincides with their TEM images. As shown in Figure S5a-e, MgF2 components transfer from shell structure to nanoparticles (a great number of particles in Figure S5e) gradually, which indicates that the defects of MgF2 compositions decrease and its crystallinity becomes higher with the reduce of Mg:F to 1:2. Figure S5c and d show that the good crystallinity of MgF2 is not beneficial for the formation of VO2@MgF2 structure, and the suitable molar ratio of Mg:F is 1:0.5. Below the stoichiometric ratio of F element in MgF2 was also reported to prepare dense and raspberry hollow particles.18 Fourth, the influence of different injection rates of NH4F components on VO2@MgF2 formation with 0.2, 0.6, 1.2, and 2 mL/min (concentration of solution is 0.12 M) was characterized by TEM in Figure S6. As shown, the slow injection rate corresponds to generate long strip floccules, however, with the injection rate accelerates, the strips change gradually to dense shell covered on the surface of VO2 nanoparticles. It is evident that the uniform, continuous, and dense VO2@MgF2 core-shell nanoparticles just obtained within a narrow synthesis conditions in the solvent of deionized water from the above results. The strict conditions could be attributed to the low viscosity of water and fast F− diffusion rate, which makes the transformation from MgF2 monomer to nucleation and subsequent growth rapidly. This procedure is so fast that the reaction cannot control well and the monomer is short of time to deposit on the surface of VO2 nanoparticles for shell growth, leading to the easy formation of MgF2 particles and precise ratio of reactants for core-shell structure with uncontrollable shell thickness. Further, the synthesis of VO2@MgF2 core-shell was moved into high viscosity solvent of EG for the regulation of MgF2 shell thickness. Figure 1b and c show the XRD patterns of as-synthesized VO2@MgF2 core-shell and MgF2 nanoparticles compared with the XRD of VO2 cores. Figure 1b indicates that the as-synthesized MgF2 nanoparticles have wide


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full width at half maximum (FWHM) and low diffraction intensity, corresponding to the small size (~5 nm) and very weak crystallinity. Two samples of VO2@MgF2 have stronger diffraction peaks than VO2 cores, and have a small peak at 27° (Figure 1b) and a bulge at 40° (the arrows pointed in Figure 1b and c), which demonstrates that the core-shell structure can strengthen the crystallinity of VO2 cores and the MgF2 shell has weaker crystallinity than MgF2 nanoparticles. Perhaps more importantly, the XRD intensity of VO2@MgF2 synthesized at 20 °C in EG is higher than that of 80 °C, and reacted supernatant at 80 °C becomes yellow but that is tintless at 20 °C. These facts are similar to the solvent of deionized water (Figure S1 and S2), which suggests that heating is not good for VO2 cores. Based on the research results in deionized water, it is successful to synthesize uniform, continuous, and dense VO2@MgF2 core-shell nanoparticles with controllable shell thickness (5-20 nm) in EG solvent at room temperature (Figure 2 and 3). Keeping the molar ratio of Mg:F fixed at 1:0.5, regulating the dosage of MgF2 components can control the MgF2 shell facilely. However, when the dosage of MgF2 components reaches the molar ratio of VO2:MgF2 = 1:1.5, there are excessive MgF2 products to form nanoparticles as shown in Figure 3c and Figure S8. Fundamentally, the high molar ratio of Mg:F and VO2:MgF2, and the low injection rate of NH4F components all correspond to the low F− concentration in the solvent at the initial stage of reaction, which makes the formation of long strip MgF2 floccules (Figure S3a and b, Figure S5a, and Figure S6a). Conversely, with the high F− concentration at the initial stage, the main products are MgF2 nanoparticles (Figure S3d, Figure S5d and e, Figure 3c), the core-shell structure forms only within these two concentrations. It needs precise F− concentration for generating core-shell structure in the solvent of deionized water at the initial stage, however, the appropriate range of F− concentration is wider in EG than that of water. As a result, the MgF2


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shell thickness can be regulated by VO2:MgF2 molar ration in EG facilely. A schematic illustration has been used for brief expounding these findings in Scheme 2.

Figure 1. (a) Crystal structure diagrammatic sketch of rutile VO2 and MgF2. (b) XRD patterns of VO2 nanoparticles, as-synthesized VO2@MgF2 at 20 and 80 °C in EG, and the synthesis of MgF2 at 20 °C in EG without VO2 cores. (c) Overlapped XRD curves of (b) from the range of 35-50°. The molar ratios of VO2:MgF2 and Mg:F were 1:1 and 1:0.5, respectively, and the injection rate of NH4F components was 2 mL/min.

Figure 2. TEM images of VO2 nanoparticles (a) and different shell thickness of VO2@MgF2 (b) and (c). The molar ratio of VO2:MgF2 was 1:0.5 (b) and 1:1 (c). Mg:F = 1:0.5, and the injection rate of NH4F components was 2 mL/min.


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Figure 3. SEM images of different shell thickness of VO2@MgF2. The molar ratios of VO2:MgF2 were 1:0.5 (a), 1:1 (b), and 1:1.5 (c).

Scheme 2. Schematic representation about the influence of reaction parameters for depositing MgF2 on the surface of VO2 core. The durability of core-shell nanoparticles. For the durability characterization of inorganic core-shell nanoparticles, the temperature-responsive optical films were prepared on the PET substrate and subsequently placed in damp heating environment (T=60 °C, RH=90%) for testing their optical properties at predetermined time intervals. Figure 4 exhibits the Vis-IR transmittance of VO2 nanoparticles and optimal inorganic core-shell nanoparticles prepared in water and EG after different ageing time. As shown, the transmittances of room temperature and high temperature get close to each other with the rise of treatment time, which reveals the invalidation of optical performance of VO2 material. Figure 4b shows that the durability of VO2@MgF2 core-shell architecture is worse than VO2 core only in Figure 4a. The reason could


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be ascribed to the hydration effect of VO2 core surface in the preparation process of core-shell. Because the VO2 nanoparticles can be damaged absolutely after several days soaked in water (Figure S9), and the broken process will be accelerated if the pH of water is not neutral. In VO2@MgF2 synthesis, the pH of reaction liquid goes to 4 after the addition of MgCl2·6H2O (aq.). Fortunately, this problem can be overcome in EG solvent, as shown in Figure 4c, the durability of VO2@MgF2 has been enhanced more than ~6.5 times (from 77.5 h to more than 500 h). The change of transmittance of these samples has a good agreement with the transmittance difference (ΔT) at λ=1200 nm in Figure 4d, the greater changes of transmittance rising correspond to the lower ΔT value which is a vital parameter for evaluating the optical performance of VO2 material.

Figure 4. Vis-IR transmittance spectra of VO2 core (a), VO2@MgF2 prepared in water (b), and VO2@MgF2 prepared in EG (c) placed at constant temperature (60 °C) and humidity (90%). The


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solid lines have been measured at room temperature, and the dash lines correspond to the spectra at 80 °C. (d) Curves of visible light transmittance and the transmittance difference (ΔT) between room and high temperature at λ = 1200 nm of these samples with the change of treatment time. From the above results, the durability enhancement of VO2@MgF2 inorganic core-shell architecture is limited, the considerable reason is the existence of cracks in inorganic shell (i.e., not completely tight). Thus, the strongly adhesive PDA has been introduced into our system for further enhancing the durability. Figure 5a shows the FTIR of optimal VO2@MgF2 (VO2:MgF2 = 1:1), VO2@MgF2@PDA, and PDA. All absorption peaks of VO2@MgF2@PDA are composed of that of VO2@MgF2 and PDA, which supports the existence of PDA on the VO2@MgF2 nanoparticles surface. The broad and strong band of PDA (Figure 5a) at approximately 3400 cm−1 was assigned to the N-H stretching vibration mode,23 and the peak at ≈1500 cm−1 was due to the benzene ring from PDA.24 And the TEM image of VO2@MgF2@PDA expresses that two well-defined shells coated on the VO2 core (see Figure S11). The optical durability of VO2@MgF2@PDA inorganic-organic core-shell was recorded in Figure 5b under the same conditions as VO2@MgF2. It is clear that the durability has been greatly enhanced after the coating of PDA, and the value of ΔT changed within 20% (from 25% to 20.1%) after the treatment time of 1000 h in constant temperature (60 °C) and humidity (90%). In terms of the durability time, VO2@MgF2@PDA has been enhanced more than ~13 times in contrast to the pristine VO2. This technique realizes the same enhancement effect as prior reported VO2@ZnO core-shell structure prepared at 85 °C for 8 h.15 While zinc oxide shell has good stability for resisting damp and heating environment, it still cannot resist strong acid, alkali, and some harsh environments. A schematic illustration of pure inorganic and inorganic-organic coated architecture for durability enhancement is shown in Figure 5c.


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Figure 5. (a) FTIR for determining the existence of PDA in samples. (b) Optical durability of VO2@MgF2@PDA inorganic-organic core-shell, the inset is the change of visible light transmittance and the transmittance difference (ΔT) with the treatment time. (c) A diagram for describing the effect of durability enhancement by inorganic-organic core-shell architecture. Flexible near-IR modulator for non-contact temperature-field fluorescence imaging. According to the temperature-responsive IR modulation ability of monoclinic VO2, we aimed to prepare stable and durable flexible near-IR modulator at the regulation range of 1000-1500 nm (scarce flexible modulator at this range) based on the durability-enhanced VO2@MgF2@PDA nanoparticls. The regulation ratio was 36.4% at λ = 1200 nm when its low transmittance at high temperature was as low as ca. 5% (Figure 6a), and this ratio declined gradually with the blueshift of wavelength. Figure 6b recorded the five cycles of heating and cooling about the transmittance of VO2-based modulator at λ = 1200 nm, which reveals the repeatable cycles about the temperature-responsive low and high transmittance of this modulator. Moreover, after undergoing 2×103 switching cycles, this regulation performance exhibits almost without declining sign, as shown in Figure S12. The fluorescent material was used to intuitively display the modulation effects of this flexible modulator here. To the best of our knowledge, the most efficient upconversion fluorescent material which can produce visible light under the excitation of near-IR light is 20%Yb3+ and 2%Er3+ co-doped NaYF4 currently under the excitation


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wavelength of 980 nm (the synthesis of this material sees Supporting Information).25-28 Here, this upconversion material was utilized for transforming the signals to visual light. As shown in Figure 6c, the differential scanning calorimetry (DSC) curves of VO2@MgF2 sample have the endothermic peak at 64.1 °C in the heating process and the exothermic peak at 41.3 °C in the cooling process, and this DSC cycle corresponds to one cycle of high-low-high transmittance in Figure 6b. The change of signals was displayed by the excitation of a dot 980 nm laser to the upconversion film in the inlaid photographs of Figure 6c. This apparent difference of fluorescence intensity can be attributed to the change of the upconversion fluorescence spectra (Figure S13). The designed optical path was illustrated in Figure 6d, the light source went through the VO2-based modulator which can control the transmittance of light with the change of temperature to upconversion film obtaining the visual light for observer. On the basis of this design, the temperature-responsive fluorescent flexible sensor can be applied to judge the uniformity of temperature-field distribution on the complicated and campulitropal surfaces remotely and swiftly due to the intrinsic lag effect of VO2 phase transformation, which is different from general point temperature measurement method. The photographs of fluorescence imaging in dark on the two surfaces which was placed at different temperature were shown in Figure 6d (inserted picture). If the sensor located on the uneven temperature field surface, the fluorescent intensity is not consistent on the whole surface. This promising viewpoint was proposed in this work first, and it still needs to modify the excitation wavelength of upconversion materials because the excitation wavelength of 980 nm does not match well with the wavelength of VO2-based modulator which has large ΔT value.


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Figure 6. (a) Vis-IR transmittance spectra of flexible VO2-based near-IR modulator film, the inset is the photograph of this flexible film. (b) The transmittance of flexible film at λ = 1200 nm after heating (~5 °C/min) and cooling (~10 °C/min) five cycles. (c) DSC curve of VO2@MgF2, the inserted pictures are the fluorescence photograph of the flexible sensor by the excitation of a dot 980 nm laser (power density of 0.5 W/cm−2) at the corresponding temperature. The photograph that is no arrow pointed corresponds to the luminescence of upconversion layer without VO2-based modulator. (d) The illustration of the designed optical path, the inserted picture is the fluorescence imaging of two surfaces at different temperatures of 20 °C (top) and 60 °C (bottom) by the excitation of a circular 980 nm laser (radius of 1 cm, power density of 1 W/cm−2). 4. CONCLUSION Here, VO2@MgF2 core-shell nanoparticles were synthesized successfully at room temperature via the investigation of the reaction solvent, temperature, the molar ratio of Mg:F and


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VO2:MgF2, and the injection rate of NH4F components in details. It was found that the durability-enhanced VO2@MgF2 with controllable shell thickness (5−20 nm) can be obtained in the solvent of EG. Further, after the coating of a PDA organic shell to form VO2@MgF2@PDA inorganic-organic core-shell architecture, its optical durability can be greatly enhanced more than 13 times in contrast to the pristine VO2, meanwhile the value of ΔT changed within 20% (from 25% to 20.1%) after the treatment time of 1000 h in constant temperature (60 °C) and humidity (90%). Additionally, the as-synthesized durability-enhanced nanoparticles were fabricated into simple flexible sensor for temperature-field fluorescence imaging through the combination of VO2-based near-IR modulator with the upconversion fluorescence material. The viewpoint of inorganic-organic core-shell architecture is also a good strategy for durability enhancement of other unstable materials, and the VO2-based flexible temperature-fluorescence sensor is a promising technique for remote and swift temperature-field distribution imaging on the complicated and campulitropal surfaces. ASSOCIATED CONTENT Supporting Information The characterization results of XRD, TEM, and supernatant photographs in the synthesis process of VO2@MgF2 with the solvent of deionized water. SEM images of VO2 nanoparticles and the synthesized MgF2 nanoparticles at room temperature in EG without VO2 cores. Optical transmittance spectra of VO2@MgF2 and the fluorescence spectra of flexible sensor at different temperature. AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] (A.H.) *E-mail: [email protected] (S.J.) Author Contributions This manuscript was written through contributions of all authors. And all authors have approved to the final version. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This study was financially supported by the National Natural Science Foundation of China (NSFC, Nos. 51372264, 51572284) and the Shanghai Sailing Program (No. 17YF1429800). REFERENCES 1.

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Abstract graph


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Room-temperature Synthesis of Inorganic-organic Hybrid Coated VO2

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