Preparation, optical properties, and thermal stability of polyvinyl butyral composite ﬁ lms containing core ( lanthanum hexaboride ) – shell ( titanium dioxide )- structured nanoparticles

: Nano - sized lanthanum hexaboride ( LaB 6 ) @ titanium dioxide ( TiO 2 ) particles with a core – shell struc ture has been successfully synthesized via a simple sol – gel method. LaB 6 @TiO 2 particles were used as ﬁ ller in polyvinyl butyral ( PVB ) matrix and performance of the TiO 2 shell was evaluated. The core – shell nanoparticles were characterized for morphology and structure proper -ties. X - ray di ﬀ raction and transmission electron micro scope testing results con ﬁ rm the formation of LaB 6 – TiO 2 core – shell structure. In composite ﬁ lm, LaB 6 improved the thermo - decomposing temperature of PVB matrix from 369.2 to 372.8°C, while the same amount of LaB 6 @TiO 2 further increased the temperature to 381.0°C. In addition, TiO 2 shell redshifted the maximum transmittance of the ﬁ lm from 605 to 669 nm in the visible region. In the near infrared region, its absorption peak shifted from 1,466 to 1,476 nm. This result will be helpful for the development of transparent and thermal insulating materials.


Introduction
As a thermionic electron emitter material, LaB 6 is known for its excellent thermal stability, creep resistance, low work function, and so on. Up to now LaB 6 is found to possess higher electronic emissivity than other materials and therefore, it has been extensively applied in various areas, such as electron lithography, high resolution optical system, and coating film for resistor [1][2][3][4].
Surface plasmon resonance refers to collective oscillations of metallic electrons at the metal-dielectric interface. The metal-like plasmonic material lanthanum hexaboride (LaB 6 ) exhibits strong surface plasmon resonance absorption in the near infrared (NIR) region after being reduced to the nanoscale [5,6]. It indicates that LaB 6 absorbs the NIR light, while maintaining high transparency in visible region. Hence, LaB 6 acts as effective component in functional materials to impair heat transmission [7]. And now, different forms of the nanoparticles (NPs) like filler or coating of polymer films have found applications in glass curtain wall and automotive glass [8,9].
During the early 1990s, researchers prepared concentric multilayer semiconductor Nps in order to improve materials' properties. Since then, materials with a coreshell structure have attracted much attention [10][11][12]. This kind of materials could be synthesized by combining many components to form various cores and shells. These nanoscaled particles have many advantages over single components, such as wider applicability, better tunability, and higher sensitivity. Indeed, extensive research has been conducted on these materials. Kim et al. found  that Ag/TiO 2 core-shell nanowires imparted exquisite resistance switching behavior to flexible, composite, and the ultra-thin shell in NPs and enhanced thermal stability of Ag core, while effectively blocking recombination of excitons [13]. Thus, this material was efficient in the disinfection of water under solar light irradiation due to distinctive surface plasmon resonance and electron charge transfer [14]. Controllable rutile titanium dioxide (TiO 2 ) shell in anatase@rutile core@shell TiO 2 nanosheets improved ethanol sensing performances [15]. Obviously, TiO 2 shell has attracted a great deal of interest in many fields due to its excellent chemical stability, photochemical activity, antibacterial activity, and so on [16][17][18][19].
More importantly, TiO 2 helps LaB 6 NPs distribute uniformly in polyvinyl butyral (PVB) due to electrostatic attraction formed at the interfaces between TiO 2 and LaB 6 [20]. So we expected to integrate LaB 6 core and TiO 2 shell to investigate how the shell would act in the core-shell structure and relevant properties. In the present study, PVB-based hybrid nanocomposite films were prepared by using core-shell structured LaB 6 @TiO 2 NPs as fillers. The effect of the core-shell NPs on optical and thermal properties of the nanocomposite films will be introduced in this article.

Preparation of core-shell LaB 6 @TiO 2 NPs
According to Chen et al. research [21], LaB 6 NPs need to be pretreated with an anionic surfactant dodecylbenzenesulfonic acid to achieve a stable dispersion of LaB 6 in ethanol. In brief, LaB 6 particles were mixed with ethanol solution of dodecylbenzenesulfonic acid (0.05 wt%), and the content of LaB 6 NPs was fixed at 1 wt%. Then, the mixture was stirred for a few minutes until a navy blue suspension was obtained. In order to achieve better homogenization, it was subsequently milled in a planetary ball mill for 24 h. A sol-gel-assisted hydrothermal method elaborated elsewhere [22] was used for preparation of LaB 6 @TiO 2 NPs. At first, 20 mL of the navy blue suspension, 2 mL of NH 3 ·H 2 O, and 2 mL of tetrabutyl orthotitanate were added to 100 mL of ethanol-water (with volume ratio of ethanol/water = 9/1). Then, the obtained mixture was kept in a water bath at 40°C under vigorous stirring for 6 h. In this process, amorphous TiO 2 shells were formed on surfaces of LaB 6 NPs. Subsequently, a hydrothermal process was used to prepare LaB 6 @TiO 2 NPs with anatase TiO 2 shells. The suspension was transferred into a Teflonsealed autoclave and then placed in an oven at 160°C for 6 h. The products were collected by centrifugation and washed with distilled water and absolute ethanol several times.

Preparation of LaB 6 @TiO 2 -PVB and LaB 6 -PVB nanocomposite films
Preparation of LaB 6 @TiO 2 -PVB composite films consisted of three steps. First, PVB powders were dissolved in ethanol via continuous stirring at 65°C for almost 1 h, and volume of ethanol was fixed at 50 mL throughout. Thus, ethanol solution of PVB was obtained. Second, LaB 6 @TiO 2 particles and silane coupling agent KH570 were mixed in ethanol under vigorous stirring in an ultrasonic bath. KH570 aimed to promote the compatibility and dispersion of LaB 6 @TiO 2 in PVB matrix. After complete dispersion, a stable navy blue suspension appeared. Finally, the suspension and an ethanol solution of PVB (10 wt%) were vigorously stirred at 65°C for 6 h (including 30 min of ultrasonic treatment). The as-obtained mixture was then cast onto slides by dip-coating method, and subsequently dried at 40°C for 24 h. After drying, films were removed from slides. Thickness of the films was fixed between 0.15 and 0.3 mm. A schematic diagram for fabrication of LaB 6 @TiO 2 NPs and LaB 6 @TiO 2 -PVB nanocomposite films was presented in Figure 1. LaB 6 -PVB films were produced when the fillers were LaB 6 NPs.

Measurements
Morphology of LaB 6 @TiO 2 NPs was determined by Transmission electron microscope (FEI-TEM; Tecnai G2 F20 S-TWIN) equipped with energy dispersive X-ray spectroscopy.
For the TEM measurement, LaB 6 @TiO 2 NPs were dispersed in ethanol and then transferred onto a copper grid covered with carbon film. XRD patterns of LaB 6 , TiO 2 , and LaB 6 @TiO 2 NPs were recorded by D-MAX2500 (Rigaku) with Cu Kα radiation (λ = 0.1542 nm and step size = 0.02°) at a scanning rate of 8°/min in the 2θ range of 15-80°. Nicolet-6700 spectrophotometer (USA) with 4/cm resolution was used to measure the FT-IR spectra of PVB and nanocomposite films in the range of 4,000-400/cm. The UV-Vis-NIR transmission spectra of 0.30 wt% LaB 6 -PVB film and 0.30 wt% LaB 6 @TiO 2 -PVB nanocomposite films were detected by a UV-vis-NIR spectrophotometer (Japan, Shimadzu UV-3600) in a wavelength range of 380-2,000 nm. The neat PVB films were set as a blank background. Thermogravimetric findings (TGA) and differential scanning calorimeter (DSC) curves of neat PVB and nanocomposite films were recorded by a STA449C simultaneous thermal analyzer at a heating rate of 10°C/min from 25 to 700°C under N 2 atmosphere.  . 89-4921), respectively. Since LaB 6 NP was enveloped with TiO 2 shell by a sol-gel hydrothermal process, diffraction peaks of the core-shell structured particles were compatible with cubic LaB 6 and anatase TiO 2 . In addition, no other diffractive peaks appear in the pattern. Neetu [23] and Liu et al. [24] also reported hydrothermal synthesis of anatase TiO 2 shell, and their XRD patterns are consistent with ours. It is concluded that  the sol-gel-assisted hydrothermal method to prepare the LaB 6 @TiO 2 NPs with high purity is feasible.

Morphology analysis
The size and structure of the core-shell-structured LaB 6 @TiO 2 NPs is determined using TEM characterization technique and the images are shown in Figure 3. It is clearly visible in Figure  3a that many particles with an average diameter of 80 nm are aggregated and there is a shielding of core particles by the TiO 2 shell. Figure 3b presents an electron diffraction (ED) pattern of the LaB 6 @TiO 2 NPs. The diffraction rings are attributed to (100) and (110) planes of LaB 6 (PDF#-34-0427) and (101) and (103) planes of TiO 2 (PDF#-89-4921), respectively. In order to detect the elements of the composite NPs, samples were analyzed by EDX, and the results are displayed in Figure 3c and d, corresponding to spots marked by the square and the circle in Figure 3a, respectively. As shown in Figure 3c, Ti peaks which are attributable to TiO 2 shell formed in the sol-gel method are significantly distinguished from other peaks, and O peak is attributed to TiO 2 and residual dodecylbenzenesulfonic acid absorbed on surfaces of LaB 6 @TiO 2 core-shell NPs. Besides, no La peaks are found. As compared to Figure 3c, La peaks in Figure 3d are apparently observed due to high content of LaB 6 at this spot. However, the light element boron does not present any peaks here due to its property, or the peaks may be overlapped by carbon's. In addition, Ti, C, and O elements were also detected. Two peaks at 8-9 keV are originated from the Cu substrate used for TEM detection. Combined with the TEM image in Figure 3a, it can be obtained that the NPs are only consisted of LaB 6 and TiO 2 without any other elements, which is in good agreement with the XRD findings; and the dark LaB 6 cores are surrounded by TiO 2 shell.
A high-resolution TEM image of a single LaB 6 @TiO 2 NPs is presented in Figure 3e. The inner LaB 6 core is tightly surrounded by a loose TiO 2 layer, and the interface between the dark core and the translucent continuous shell is noticed. Most of the shells possess a thickness less than 5 nm, but a few of them are approximately 15 nm thick. Figure 3f (inset in Figure 3e) is an ED pattern of the section marked by a circle. To further demonstrate the detailed microstructure of the interface between the cores and shells, high-resolution TEM (HRTEM) and fast Fourier transmission (FFT) measurements were carried out, and data are displayed in Figure 3g. It is clearly displayed that the two different crystal growth directions which are perpendicular to their corresponding crystal planes exist, with a distinct boundary between them. The inter-planar spacing of around 0.2145 nm is attributed to the (200) plane of LaB 6 and the lattice spacing of approximately 0.1899 nm is ascribed to the (200) plane of TiO 2 . Moreover, Figure 3h and i illustrates diffractive spots of anatase TiO 2 shell and crystalline LaB 6 core, respectively. These results further confirm that there are two different crystal forms in the LaB 6 @TiO 2 NPs, coinciding with the results of the above XRD patterns.

FT-IR spectra
FT-IR spectra of the composites are exhibited in Figure 4.  [25]. For the LaB 6 @TiO 2 -PVB, the peaks at around 520/cm are characteristic peaks of TiO 2 crystals [20] and ascribable to the stretching vibration of Ti-O-Ti bonds in TiO 2 [26,27]. Tekin et al. [28] also observed Ti-O-Ti stretching vibration modes of anatase TiO 2 in a form of a broadband ranging from 647 to 830/cm. These spectra are similar in appearance, and it may be attributed to low contents of solid particles in the composite films and their weak absorption in this region.

Thermal degradation characteristics
The thermodynamic stability of the films was investigated by means of DSC and TGA ( Figure 5). As demonstrated by a TGA curve in Figure 5a, thermal degradation of neat PVB film included two steps. The first weight loss that occurred below 100°C may be resulted from water evaporation in the matrix, which was absorbed in the film due to Van der Waals force. The second weight loss that started at 250°C and finished at 400°C is probably attributed to the degradation of PVB matrix, and it exhibited a weight loss of 90.3%. However, it is obvious that PVB in LaB 6 -PVB and LaB 6 @TiO 2 -PVB films degraded at higher temperatures. Especially for LaB 6 @TiO 2 -PVB nanocomposite films, it happened at around 320°C. Additionally, LaB 6 -PVB and LaB 6 @TiO 2 -PVB nanocomposite films presented weight loss of 89.4% (Figure 5b) and 83.4% (Figure 5c), respectively. As a comparison, 0.3 wt% LaB 6 @TiO 2 -PVB film is not vulnerable to heat.
Endothermic peaks at 369.2°C (Figures 5a), 372.8°C (Figure 5b), and 381.0°C (Figure 5c) are apparently noticed in the DSC curves of neat PVB, LaB 6 -PVB, and LaB 6 @TiO 2 -PVB nanocomposite films, respectively, and they might be originated from degradation of PVB matrix. In addition, when LaB 6 @TiO 2 NPs were added into neat PVB, the decomposition temperature of the matrix almost improved from 369.2 to 381.0°C, indicating an enhancement of thermal properties of the nanocomposite films. This phenomenon should be ascribed to the strong hydrogen bonding effect between the polar hydroxyl groups of PVB and the oxygen atoms of LaB 6 @TiO 2 NPs. Similar reports have been presented by Hemdana et al. [12] and Mallakpour and Dinari [26].

UV-Vis-NIR spectra
Transmission spectra of 0.30 wt% LaB 6 -PVB and 0.30 wt% LaB 6 @TiO 2 -PVB nanocomposite films in Figure 6 were measured in a wavelength region between 380 and 2,000 nm. Both samples show high transmittance in visible region (380-780 nm), and strong absorption peaks in NIR region (780-1,500 nm), and their transmittance curves in the spectra are similar in appearance. Additionally, transmittance spectra of LaB 6 -PVB and LaB 6 @TiO 2 -PVB nanocomposite films are almost the same with a maximum absorption in the region of 1,000-1,500 nm. In contrast, LaB 6 @TiO 2 -PVB nanocomposite films illustrated better transmittance in visible region.
The peak transmittance of LaB 6 -PVB and LaB 6 @TiO 2 -PVB film were located at 605 and 669 nm, respectively.  It means that an anatase TiO 2 shell enables the peak transmittance to redshift for around 64 nm in visible region, and the absorption peak in NIR region to shift slightly from 1,466 to 1,476 nm. It might be attributed to enlarged diameter of LaB 6 @TiO 2 NPs by TiO 2 shell. As has been reported in a previous research, enlarged diameter of a core-shell structure enabled absorption peak and transmission peak to redshift [12]. Thereby, it provides us a promising method to tune the wavelength range of the visible light transmission without any effect on the absorption of the NIR light.
To the best of our knowledge, films with high transmittance in the visible region and low thermal insulating properties have great application potential in automobile glasses. Based on the above results, the LaB 6 @TiO 2 -PVB nanocomposite films could fulfill these two requirements and therefore, would be a potential candidate for transparent heat-insulated materials in some applications.

Conclusion
LaB 6 @TiO 2 NPs with a core-shell structure have been synthesized via a sol-gel-assisted hydrothermal method. XRD characterization shows that the NPs are composed of cubic LaB 6 and anatase TiO 2 . TEM results confirm that the NP is composed of a cubic LaB 6 core and an anatase TiO 2 shell. FT-IR and TGA analyses illustrate that the thermal stability of the composite was enhanced by the TiO 2 shell due to the interaction between the NPs and PVB matrix. In LaB 6 @TiO 2 -PVB nanocomposite films, an anatase TiO 2 shell apparently expands the wavelength range of the visible light transmission of the composite films. This work provides us a comparatively simple method to synthesis LaB 6 @TiO 2 NPs and a feasible way to produce composite films with high light transmittance and high thermal insulation.