High performance surface-modified TiO2/silicone nanocomposite

The mismatch of refractive index (RI) between light emitting diode (LED) chips and packaging resins severely lowers the lighting emitting efficacy of LED. The RI can be enhanced by the introduction of high RI nanoparticles but meanwhile it is a great challenge to maintain the high transparency for resins due to the agglomeration of nanoparticles. In this work, a facile strategy is proposed to fabricate silicone nanocomposites with a high transparency (>88%, less than 2% decrease relative to pure silicone resin), largely enhanced RI (an increase from 1.42 to 1.60) and improved thermal stability (73 °C increase in weight loss of 50%). Specifically, the ultra-fine monodispersed TiO2/silicone composites are prepared by direct solvent mixing of 1 wt% surface modified TiO2 nanoparticles (S-TiO2) into the silicone resin, in which S-TiO2 are prepared by direct introduction of titanate coupling agent in the process of TiO2 growth to induce the formation of protective layer on the surfaces of TiO2 nanoparticles. This methodology demonstrated is simple, cost-effective and versatile for the massive fabrication of highly transparent LED packaging materials with greatly enhanced refractive index and meanwhile enhanced thermal stability.

stability of polymers. Thus, a highly transparent TiO 2 /silicone nanocomposite with a high RI and high thermal stability can be obtained by directly mixing the ultra-fine S-TiO 2 nanoparticles with the silicone resin. To evaluate their potential as LED packaging material, the transmittance, RI and thermal stability of the as-prepared silicone nanocomposite are examined. As a control, several types of TiO 2 nanoparticles without the introduction of coupling agent and the corresponding composites are prepared and compared with the surface modified S-TiO 2 case.

Results and Discussions
Synthesis of S-TiO 2 . Figure 1 shows the general steps to fabricate S-TiO 2 /silicone nanocomposites. First, the TiO 2 nanoparticles synthesized via sol-gel approach were introduced into silicone resin/acetic ether solution under ultrasonic treatment, forming a uniform suspension. Subsequently, the resulting suspension was subjected to a vacuum chamber to remove acetic ether and bubbles. Finally, the transparent nanocomposites were prepared by curing the resin at 100 °C. The only difference of the preparation procedure between S-TiO 2 /silicone nanocomposite and un-treated TiO 2 /silicone nanocomposite is the introduction of titanate coupling agent, which endows TiO 2 nanoparticles with protective layers on their surface.
The morphology of the as-prepared TiO 2 materials was characterized by TEM. As shown in Fig. 2, the TiO 2 -1 has a diameter of approximately 5 nm. Increasing the reaction time from 48 h to 72 h leads to an increase of the diameter of TiO 2 -2 nanoparticles from ca. 10 to ca. 15 nm, accompanying with TiO 2 aggregation. TiO 2 -3 obtained by drying TiO 2 -2 at 120 °C for 24 h shows a significant increase in the diameter of particles up to the scale of micrometer. By contrast, S-TiO 2 nanoparticles with a diameter of 3-4 nm are mono-dispersed in the composite. The afore-mentioned results are supported by the BET measurement. As shown in Table S1, specific surface areas of 35.8, 17.6, 2.84, and 75.6 m 2 /g are achieved for TiO 2 -1, TiO 2 -2, TiO 2 -3 and S-TiO 2 , respectively, which agrees well with the result for the diameter of TiO 2 nanoparticles. X-ray diffraction patterns (top insets of Fig. 2a-d) indicate that the crystallinity of TiO 2 increases with the increases of reaction time and temperature. Additional evidence on the chemical structure of the TiO 2 particles was provided by EDS analysis. C originated from titanate coupling agent is observed in S-TiO 2 while no trace of C is found in TiO 2 -1, TiO 2 -2 and TiO 2 -3 (Table S1), indicating formation of protective layers on the  Dispersibility of TiO 2 . To achieve a homogenous dispersion of TiO 2 in silicone resin, co-solvent is needed to dilute silicone resin due to the poor solubility of silicone resin in ethanol. As shown in Fig. 2e and f, the suspensions of S-TiO 2 and silicone resin formed in acetic ether both are transparent, while their suspensions formed in other solvents including acetone, petroleum ether and cyclohexane are turbid. In addition, acetic ether featured by low toxicity and low boiling point is a good candidate for the massive fabrication of TiO 2 /silicone nanocomposite. The dispersity of TiO 2 -1, TiO 2 -2 and TiO 2 -3 in acetic ether was also investigated via the same strategy. As presented in Table 1, all of the suspensions of TiO 2 -1, TiO 2 -2 and TiO 2 -3 formed in acetic ether are turbid, indicating that the dispersibility of S-TiO 2 in acetic ether is much better than that of TiO 2 without surface modification. In addition, to reveal the effectiveness of various surface modifiers in improving the dispersibility of TiO 2 , the transparency of their suspensions in acetic ether was further studied. As presented in Table S2, all of the suspensions of TiO 2 modified by oxalic acid, citric acid, KH-550 and KH-560 are turbid, indicating that titanate coupling agent is the most effective in improving the dispersibility of TiO 2 in acetic ether.
Transmittance of TiO 2 /silicone nanocomposites. The cross sections of silicone nanocomposites of various TiO 2 nanoparticles were observed with SEM. As shown in Fig. 3a-c, aggregates sized ca. 150 nm, 30 nm and 10 nm are observed in the nanocomposites incorporated with TiO 2 -1, TiO 2 -2, and TiO 2 -3, respectively, suggesting the poor dispersion of TiO 2 -1, TiO 2 -2 and TiO 2 -3 in the silicone resin. In comparison, no visible aggregates appear in the S-TiO 2 /silicone nanocomposite. S-TiO 2 nanoparticles are mono-dispersed in the silicone resin with a diameter around 3-4 nm (see the top inset of Fig. 3d). The narrow size distribution and excellent dispersion of S-TiO 2 in the silicone resin are attributed to the titanate coupling agent layers formed on the surfaces of TiO 2 nanoparticles, endorsing the excellent compatibility between TiO 2 nanoparticles and silicone resin, and thus eliminating the aggregation of TiO 2 nanoparticle in both acetic ether and silicone resin.
The following equation gives the light intensity of a spherical particulate composite 26, 27 :  SCIenTIfIC REPORTS | 7: 5951 | DOI:10.1038/s41598-017-05166-7 where r, n p and V p are the radius, RI and volume fraction of spherical particles, respectively. n m is the RI of matrix, λ is light wavelength, and x is the thickness of the composite. As revealed in Equation 1, the diameter of nano-filler significantly affects the transmittance of the composite. Therefore, to maintain the high transparency of the composite, the ultra-fine nano-fillers with mono-dispersion are strongly required. Meanwhile, the transmittance of a composite is highly related to the thickness of composites, it is quite difficult to achieve a high transparency for bulk materials than thin films. To our best knowledge, all of the transparent composites with enhanced RI reported previously are thin films with a thickness of 10 −8 to 10 −4 m, whereas bulk transparent nanocomposites with a thickness of 10 −3 to 10 −2 m are rarely reported [27][28][29][30] . The thickness of the resin layer as encapsulating material surely has some effect on the lighting efficiency of LED. In this study, the thickness of the as-prepared silicone composite is ca. 1 mm (namely ca. 10 −3 m), close to that of realistic packaging materials for LEDs. This could give us practical evaluation of the current protocol on the performance of the as-prepared silicone composite. The high specific surface area of nanoparticles would lead to irreversible aggregation 32,33 . Aggregated nanoparticles usually have much larger sizes than individual nanoparticles, and thus would have an adverse effect on the transparency and lighting efficiency of LED. In this work, S-TiO 2 nanoparticles are mono-dispersed in the silicone resin, and thus the resultant composite as encapsulating material has a high transparency (Fig. 4). As a result, the introduction of S-TiO 2 would have no adverse effect on the efficiency of LED. Figure 4a shows the transmittance of the pure silicone resin and silicone nanocomposites with 1 wt% TiO 2 . Compared with the pure silicone resin, only 2% decrease in transmittance is achieved in the S-TiO 2 /silicone nanocomposite. As the TiO 2 particle size increases, the transparency of silicone nanocomposite decreases. Figure 4b visually demonstrates the transparency of the corresponding silicone nanocomposites. The transparency of TiO 2 -1/silicone nanocomposite and TiO 2 -2/silicone nanocomposite is approximately 83% and 75%, respectively. Further increase in the particle size of TiO 2 results in an opaque silicone nanocomposite with 20% of transmittance in the visible light range. This change obeys Equation 1.
The effect of S-TiO 2 filler content on the transmittance is shown in Fig. 4c. With the addition of 0.5 and 1.0 wt% S-TiO 2 , the transmittance of silicone nanocomposite is close to that of pure silicone resin, and the average decrease of the transmittance in the visible range of 400-800 nm is less than 2%. As revealed in Fig. 4d, no  where η encapsulated and η unencapsulated are the lighting efficiency of encapsulated and un-encapsulated LED chip, respectively; n pm and n chip are the refractive index of packaging materials and LED chip, respectively. It is understandable from Equation 2 that as the RI of the packaging material increases, the lighting efficiency first increases quasi-linearly, then sub-linearly, and finally reaches a saturation. Since the RI of conventional lighting chip material is 2.5~3.0, the packaging material with a higher RI will lead to a higher lighting efficiency of LED. In this work, the introduction of fine TiO 2 particles with a high RI brings about a high RI of the composite as encapsulating material, aiming at increasing the lighting efficiency of LED.
The RI measured for the silicone nanocomposite with 1 wt% S-TiO 2 , TiO 2 -1, TiO 2 -2, and TiO 2 -3 is 1.596, 1.596, 1.602, and 1.642, respectively. The RI of silicone nanocomposite filled with TiO 2 -3 is somewhat higher than other nanocomposites, due to the crystallinity of TiO 2 -3 formed with longer aging time is better than that of S-TiO 2 , TiO 2 -1, and TiO 2 -2. Figure 5 shows the RI of S-TiO 2 /silicone nanocomposites as a function of S-TiO 2 mass fraction. As the S-TiO 2 loading increases from 0 to 0.5 and 1 wt%, the RI of silicone resin is dramatically increased from 1.42 to 1.56 and 1.596, achieving the improvement of 9.8% and 12.3%, respectively. In comparison with other researches, the current strategy shows higher improvements in RI. For instance, the incorporation of 1% of silane modified TiO 2 and oleic acid capped TiO 2 in to silicone resin increases the RI of silicone composite from 1.51 to 1.56 and 1.575, respectively 32,33 . As the content of S-TiO 2 increases further, the increasing rate of the RI is declined. Less than 2% of increase in RI is achieved as the content of S-TiO 2 increases further to 5 wt%, indicating that the effectiveness of RI enhancement at a higher filler content is limited. In principle, the porosity if any would reduce the RI of the composite since the air has a RI of unity. In the current work, the porosity measured on the surfaces of TiO 2 particles was around 3%. On the other hand, silicone resin might penetrate into the porosity. As a result, the effect of the porosity on the RI could be neglected. Therefore, the RI of the composite was determined mainly by TiO 2 particles and enhanced by the incorporation of TiO 2 particles as confirmed by Fig. 5. Thermal stability of TiO 2 /silicone nanocomposite. Figure 6 shows the TGA curves of the pure silicone resin and the silicone nanocomposites with various S-TiO 2 loadings. Although the starting decomposition temperature is almost the same for the pure silicone resin and the silicone nanocomposites, the incorporation of TiO 2 nanoparticles decreases the decomposition speed of the silicone nanocomposite. Taking the example of the weight loss of 50%, the temperature of the silicone nanocomposites with 0%, 1% and 5% S-TiO 2 is 532, 605 and 632 °C, respectively; namely, the temperature corresponding to the weight loss of 50% has been increased by 73 and 100 °C, respectively. The "cross-link density" of the nanocomposites is increased with the incorporation of TiO 2 nanoparticles, which leads to the enhancement of their thermal stability with a decreased decomposition rate 34 .

Conclusions
In summary, a facile approach to produce highly transparent TiO 2 /silicone nanocomposite with a high RI and a high thermal stability has been demonstrated by the incorporation of S-TiO 2 nanoparticles treated by titanate coupling agent into diluted silicone resin via simple solvent mixing. The titanate coupling agent layers formed on the surfaces of ultra-fine 3-4 nm TiO 2 nanoparticles endorses the excellent dispersion of S-TiO 2 nanoparticles in the silicone resin and an excellent compatibility between the TiO 2 nanoparticles and the silicone resin. Consequently, with the addition of 1 wt% S-TiO 2 , the as-prepared silicone nanocomposite shows an enhanced RI from 1.42 to ca. 1.60 (13% enhancement), a high transmittance of 88% in the entire visible light range for 1 mm thick samples and meanwhile an enhanced thermal stability. The current strategy is cost-effective, environmental-friendly, and easy-processing, which has a great potential to be widely used in producing transparent packaging materials with the greatly enhanced RI and meanwhile a high thermal stability. Synthesis of S-TiO 2 nanoparticles. S-TiO 2 nanoparticles were prepared with a typical sol-gel procedure. First, 0.01 M tetrabutyl titanate was added to 50 mL ethanol, then the solution was stirred for 15 min to ensure complete dissolution of tetrabutyl titanate. Afterwards, 0.12 mL TM-P was introduced into the suspension and stirred for another 15 min. Subsequently, 1 mL distilled water was added to the suspension obtained and kept stirring for 48 h at 40 °C.

Materials.
In parallel, TiO 2 nanoparticles with different sizes were synthesized using the similar protocol in the absence of the titanate coupling agent TM-P. The samples react for 48 h and 72 h, denoted as TiO 2 -1 and TiO 2 -2, respectively. TiO 2 -3 was prepared by vacuum drying of TiO 2 -2 at the temperature of 120 °C for 24 h.
Fabrication of TiO 2 /silicone nanocomposites. In a typical fabrication procedure, 10 g silicon resin with the mass ratio of 10:1 for Part A and Part B was dissolved in 20 mL acetic ether by ultrasonic treatment until a transparent solution was obtained. Then, 0.11 g TiO 2 particles (TiO 2 -1, TiO 2 -2, TiO 2 -3 or S-TiO 2 ) were homogenously mixed with silicone resin/acetic ether solution by ultrasonic treatment. The bubbles and acetic ether were removed by vacuum pump. Finally, the resultant suspension was casted into a stainless steel mould and then transferred into an oven at 100 °C for 2 h. The thickness of as-prepared silicone composites is approximately 1 mm.
Characterizations. The transmittance of the nanocomposite was obtained from a Hitachi U-3900. The RI was measured on an Abbe Refractometer (WAY-2S) in ambient atmosphere. Transmission electron microscopy (TEM) images and electronic diffraction were taken on a JEM-2100F instrument (operated at 200 kV). Scanning electron microscopy (SEM) images and energy dispersive spectroscopy (EDS) were collected on a Hitachi S-4300. The Brunauer-Emmett-Teller (BET) specific surface areas were calculated using adsorption data in P/P0 = 0.05-0.3 (six points collected). The weight loss analysis of the samples was conducted on a Netzsch STA 409 PC/PG at the rate of 10 °C/min under nitrogen atmosphere. Fourier transform infrared (FTIR) spectra were recorded using a Varian 3100 FT-IR spectrometer with 2 cm −1 resolution and accumulation of 24 scans.