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High-gravity-hydrolysis approach to transparent nanozirconia/siliconeencapsulation materials of light emitting diodes devices for healthy lightingXianglei Hea,b, Ruijie Tanga,b, Yuan Pua,b, Jie-Xin Wanga,b, Zhi Wanga, Dan Wanga,b,∗,Jian-Feng Chena,b

a Beijing Advanced Innovation Center for Soft Matter Science and Engineering, State Key Laboratory of Organic Inorganic Composites, Beijing University of ChemicalTechnology, Beijing, 100029, Chinab Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing, 100029, China

A R T I C L E I N F O

Keywords:High-gravity technologyZirconia nanoparticlesNanocompositeLED encapsulationBlue light

A B S T R A C T

The white light emitting diodes (LEDs) devices based on the use of blue chips and yttrium aluminum garnetphosphors have great potential impact on energy savings in the world. However, there has been public concernover the non-visual influences of blue light on human health. Herein, we report a high-gravity-hydrolysis ap-proach for the synthesis of transparent nanozirconia/silicone hybrid materials, which can be used as the en-capsulation of white LEDs with enhanced light extraction efficiency and reduced blue light exposure. The for-mation of zirconia nanoparticles on silicone chains is performed in a high-gravity rotating packed bed (RPB)reactor. The homogeneous micromixing of the reactants in the RPB enables the formation of ultrasmall zirconiananoparticles in the silicone chains. Compared with LEDs encapsulated with commercial silicone, the white LEDspackaged by nanozirconia/silicone exhibit significant enhanced luminous flux (from 13.28 to 31.34) and de-creased proportion of blue light (from 3.1% to 1.8%), resulting in low correlated color temperature (4573 K).This work demonstrates a scalable and efficient approach for healthy lighting based on the commercial whiteLEDs technology.

1. Introduction

The light-emitting diodes (LED) devices based on the use of semi-conductor chips to convert electricity into light, cut energy use by morethan 80% than conventional incandescent lights [1–4]. Therefore, thewidespread use of LED lighting has the greatest potential impact onenergy savings in the world. To date, most commercial white LEDs isfabricated by coating yttrium aluminum garnet (YAG) on the chip ofblue LEDs. The YAG phosphors emit yellow light excited by the bluelight and their combination yields white light [5]. The high blue lightratio in the emission spectrum of such LEDs is controversial, and anexcessive amount of blue light has been considered to be harmful to thehuman body [6,7]. The white LEDs with low blue light ratio for dailylighting has won broad acceptance as healthy lighting [8]. Some ofpromising methods for reducing the blue light ratio in the LEDs is toadd a certain amount of red phosphors to improvement of color ren-dering index [9–11] and/or blocking films to reduce the blue lightexposure [12]. However, the complicated production procedure limitsthe commercialization application of these methods [13–15]. Recently,

a few studies have demonstrated that the addition of nanoparticles inthe packaging layers can reduce the blue light ratio in the emissionspectrum of white LEDs [16,17].

On the other hand, the light of LEDs is emitted from the chipthrough the encapsulating material layer into the air. Due to the dif-ference between the refractive index (RI) of semiconductor chips (> 2.4for blue chips of gallium nitride) and the RI of encapsulating materials(1.5–1.6 for commercial silicone), the total reflection occurs at the in-terface will greatly reduce the light extraction efficiency [18,19]. Alongwith others, we have demonstrated that the addition of inorganic na-noparticles with high RI to bridge the gap of RI between the chipsthrough encapsulation materials to the air is an effective way to im-prove the light extraction efficiency of LEDs devices [18–22]. The ZrO2

nanoparticles in particular, are excellent candidates as fillers forpolymer nanocomposites to achieve a tunable refractive index [22–24].However, the commercial silicone used for LEDs packaging are gen-erally oligomers (especially commercial-grade two-part optical resins),which are less compatible with nanoparticles. The surface modificationof ZrO2 nanomaterials enables them to be dispersible in liquids or solid

https://doi.org/10.1016/j.nanoen.2019.05.024Received 10 April 2019; Received in revised form 29 April 2019; Accepted 8 May 2019

∗ Corresponding author. Beijing Advanced Innovation Center for Soft Matter Science and Engineering, State Key Laboratory of Organic Inorganic Composites,Beijing University of Chemical Technology, Beijing, 100029, China.

E-mail address: wangdan@mail.buct.edu.cn (D. Wang).

Nano Energy 62 (2019) 1–10

Available online 10 May 20192211-2855/ © 2019 Elsevier Ltd. All rights reserved.

T

polymers but the complicated production procedure of the surface en-gineering are not suitable in moving the ZrO2 based hybrid en-capsulation materials toward commercial products [19]. The majorchallenge for scalable production of ultrasmall nanoparticles in liquidsolution is to achieve homogenous mixcromixing in the reactors for fastand uniform crystallization [25,26]. Compared with conversionalbatch-type stirred tank reactors (STR), the high gravity rotating packedbed (RPB) reactors allow much faster mass transfer and molecularmixing of the reactants [27,28], which have been applied for produc-tion of various types of micro- and nanoparticles via nanoprecipitationin liquid solutions coupled with hydrothermal or annealing treatments[29–34].

In this work, we propose a high-gravity-hydrolysis approach tosynthesis ultrasmall ZrO2 nanoparticles in commercial-grade siliconeresins. The high gravity rotating packed bed (RPB) reactor was used tointensify the micormixing of the reactants, which enabled the in situformation of ultrasmall zirconia nanoparticles in the silicone chains.The hybrid materials were investigated by both dynamic light scat-tering (DLS) analyzer and transmission electron microscopy (TEM). Thenanozirconia/silicone hybrid materials exhibited high transparency oflight (> 98%) and large RI up to 1.63. The white LEDs devices werefabricated by using commercial blue chips, YAG phosphors and thenewly developed hybrid encapsulation materials. Compared with theLEDs encapsulated with commercial silicone, the LEDs devices pack-aged by nanozirconia/silicone exhibit significant enhanced luminousflux and decreased proportion of blue light, resulting in low correlatedcolor temperature. This work demonstrate a scalable and efficient ap-proach for developing healthy lighting devices based on the existentLEDs technology eliminate the use additional red phosphors.

2. Experimental section

2.1. Materials

Zirconium (IV) propoxide (ZPP, ca. 70% w/w in 1-propannaol), 2,4-pentanedione (Hacac), tetrahydrofuran (THF), toluene, 1-butanol(BuOH), methyl ethyl ketone (MEK) and acetic acid (AcOH) werepurchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China).The commercial-grade silicone encapsulant (OE-6336) from DowCorning was used in this study, which was supplied as a two-part A/Bkit. The A-kit had a viscosity of 13000 mPa s and the B-kit had a visc-osity of 5100 mPa s. The Ce:YAG yellow phosphor (YAG-04) with acentral wavelength of 558 nm for LED encapsulation was purchasedfrom Geliang Photoelectric Co., Ltd. (Shenzhen, China). All chemicalswere used without further purification. Reverse osmosis water (ROwater) prepared by a Hitech laboratory water purification systemDW100 (Shanghai Hitech Instruments Co., Ltd) was used for all ex-periments.

2.2. Synthesis of ZrO2 nanoparticles in the A part resin by the RPB reactor

Brief, certain amounts of ZPP and AcOH as well as 0.5 g A-kit resinwere added to a solution composed of MEK and BuOH ratio of 1:1. Acertain amount of water was also added to another solution composedof MEK and BuOH ratio of 1:1. A high-gravity RPB reactor was used forthe synthesis of ZrO2/silicone nanocomposites and the schematic dia-gram of experimental setup is shown in Fig. 1. For the RPB reactor usedin our experiments, the inner radius of the packing is 0.04 m and theouter radius of packing is 0.084 m. The circulating fluid temperature inthe reactor shell was set at 4 °C to avoid the catastrophic precipitationof particles at high temperature. Typically, the two kinds of liquidswere pumped into the RPB reactor through the peristaltic pump at aflow rate ratio of 1: 1 (feed speed is 100 mL/min) and the reactionsolution was collected at the outlet of the RPB reactor. The influencingof rotation speeds of the RPB reactor was investigated at 500, 1000,1500 and 2500 rpm, respectively. The mean high-gravity levels (β)

were 18, 72, 163 and 451 at various rotation speeds of 500, 1000, 1500and 2500 rpm, respectively, as calculated by following equation[33,34].

= rg2

Furthermore, the mean high-gravity factor (β) in RPB can be cal-culated using the following equation:

= = + ++

r dr

r drr r r r

r r g2

22 ( )

3( )r

r

rr

21

21 2 2

2

1 2

12

12

Where r1 r2, and ω are inner radius of packing (m), outer radius ofpacking (m) and rotor speed (rad/s), respectively. The g is the gravityacceleration and the value of this number is 9.8 m/s2.

The comparative experiments for the in situ synthesis of ZrO2/sili-cone nanocomposites by similar routes while batch type conversionalstirred tank reactors (STR) and ultrasonic reactors (UR) were used in-stead of RPB reactor. After the reacted solution was collected, the0.5 g B kit resin was added into the solution. The solution was heated at50 °C in a rotary evaporator to remove the all solvents. In order toprevent from aggregation of ZrO2 particles, so a molar amount Hacacthat equal to the original ZPP was added into the resin. The methods forpreparing these ZrO2/AB resins with different conditions were listed inTable 1. The amount of ZrO2 nanoparticles was estimated by the rawmaterial ZPP added. Experiment groups were designed according to thisprinciple.

2.3. Curing of AB and ZrO2/AB resins

The AB and ZrO2/AB resins were diluted with toluene to a con-centration of 20 wt % for measuring their optical, dispersal, andthermal properties. The hybrid dispersion can be spin-coated (SpinCoater, KW-4A) onto a silicon slide or a glass slide to form a thin film.The parameters of the spin coater are set to 3 s (1500 rpm) for the firstperiod, 15 s for the second period (2500 rpm). Subsequently they curedat 100 °C for 30 min, and then 150 °C for 1.5 h to obtain cured ZrO2/ABhybrid films. For comparison, pure OE-6636 silicone resin was alsoprepared by directly mixing the A kit and B kit resins with equalamounts and the procedures for preparation of thin films were similaras those of ZrO2/AB resins.

2.4. Preparation of white LED devices

1 g of ZrO2/AB resins was mixed with 0.15 g of YAG-04 phosphorand uniformly stirred with a small amount of THF. The solvent was thenremoved using a rotary evaporator. The SMD (surface mounted device)LEDs chips were used for the LED test (3528, 0.2 W, epileds). Thegroove of the chip were covered by the glue layer and the final productswere obtained followed by treatment of the devices in the oven withprogrammed temperature (100 °C for 30 min, and then 150 °C for1.5 h).

2.5. Characterization

Particle size and size distribution of the ZrO2/AB dispersion liquidwere determined by DLS (Nano ZS90, Malvern). The TEM images weretaken using Hitachi H-7700 TEM for general observation and H-9500environmental TEM operating at 300 kV in the bright-field mode forhigh resolution imaging. The surface morphologies of the thin filmswere investigated by using a JEOL JSM-6360LV scanning electron mi-croscope (SEM). The transmittances of the cured ZrO2/AB thin filmswere measured by a UV–visible spectrophotometer (UV–vis, Varian,Cary 50) from 300 nm to 800 nm in wavelength. A PerkinElmer spec-trum GX Fourier transform infrared (FTIR) spectroscopy system wasused to record the FITR spectra of the samples. The haze index was

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Fig. 1. Experimental setup for in-situ synthesis of ZrO2 nanoparticles in silicone resins using RPB reactor.

Table 1Recipes for synthesizing the ZPP/AB with different ZPP contents.

Sample ZPPa [g] A kit [g] B kit [g] AcOH [g] Water [g] MEK/BuOH [1/1] [g] Reaction speed [rpm] Cycle time [min] Reactor type

ZPP/AB1 0.034 0.500 0.500 0.004 0.001 50 1000 10 RPBZPP/AB5 0.175 0.500 0.500 0.019 0.005 50 1000 10 RPBZPP/AB10b 0.370 0.500 0.500 0.040 0.012 50 1000 10 RPBZPP/AB15 0.588 0.500 0.500 0.065 0.019 50 1000 10 RPBZPP/AB20 0.833 0.500 0.500 0.092 0.027 50 1000 10 RPBZPP/AB-S 0.370 0.500 0.500 0.040 0.012 50 1000 10 STRZPP/AB-U 0.370 0.500 0.500 0.040 0.012 50 – 10 URZPP/AB-RPB1 0.370 0.500 0.500 0.040 0.012 50 500 10 RPBZPP/AB-RPB3 0.370 0.500 0.500 0.040 0.012 50 1500 10 RPBZPP/AB-RPB4 0.370 0.500 0.500 0.040 0.012 50 2500 10 RPB

a The molar ratios of [ZPP]/[AcOH] and [ZPP]/[H2O] were both set at 1/2. The solvent for 70% w/w of ZPP materials used in our experiments is 1-propannaol.b The ZPP/AB10 sample is the same as sample ZPP/AB-RPB2.

Fig. 2. The intensity (a) and number (b) of the re-sults for particle size distributions by use RPB, URand STR. (c) The number based size distributionprofiles of samples nanozirconia/silicone prepared inRPB with different high gravity levels (16, 72, 163,451, respectively). (d) The number based size dis-tribution profiles of nanozirconia/silicone by usingprecursors with different ZPP contents (1, 5, 10, 15,20 wt%).

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tested by the WGT-S transmittance haze meter. Thermogravimetricanalysis (TGA) and differential scanning calorimeter (DSC) were per-formed by using a Q50 TGA from TA Instruments. The refractive indexof the cured ZrO2/AB films was determined by ellipsometry (UVSEL,HORIBA jobin Yvon, France) in the range of 300 nm–800 nm. The lightextraction efficiency of LEDs devices was measured by the EverfineLEDspec system.

3. Results and discussion

The zirconia/silicone nanocomposites prepared by RPB reactorswere firstly characterized by DLS measurements and the samples pre-pared by conversional STR and UR were also analyzed for comparison.

As the results of the light intensity distributions of the particles shownin Fig. 2a, the particles of the STR group were much larger than that ofthe RPB group and UR group. It was noted that there were still largeparticles and aggregates in the RPB product, which was attributed tothe composite of ZrO2 particles and silicon resin. The size of singlenanoparticle can be estimated from the number distribution of particlesize (Fig. 2b), in which the average particle size of the RPB group is2–3 nm. The average number distributions of particle size are 10 nmand 88 nm for those in the UR group and STR group, respectively.Verified from the final product of zirconia/silicone nanocomposite (Fig.S1), the nanocomposites prepared by the conventional method ex-hibited whitish after stored for one week, while the zirconia/siliconeproduct obtained by RPB route were still transparent. These results

Fig. 3. (a–b) TEM and (c–d) HRTEM images of hybrid ZrO2/silicone. (e) The schematic diagram of the structure of hybrid ZrO2/silicone.

Fig. 4. (a) FTIR and high resolution FTIR spectra of resins with different ZPP contents (0, 1, 10, 20 wt%). (c) TGA and (d) DSC curves of resin with different ZPPcontents.

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demonstrated that the RPB reactor was benefit for preparing ultrasmallZrO2 particles. The results shown in Fig. 2c indicated that the averageparticle sizes of ZrO2 nanoparticles decreased as the increase of high-gravity level. However, it should be pointed out that the comprehensiveperformance of the hybrid nanozirconia/silicone encapsulation mate-rials prepared at larger high gravity level was not better. The wire meshpacking in the RPB generated a huge shear force and the violent dis-turbance in the RPB reactors working at large high-gravity level waseasy to destroy the silicon resin structure, which then affected thecuring of the encapsulation materials. For example, the surface flatnessof the film prepare by the nanozirconia/silicone prepared obtained athigh-gravity level of 451 was inferior to that obtained at high-gravitylevel of 72 (Fig. S2). Therefore, the optimized experiment parameter of

the high gravity level of the RPB reactors in our experments was fixed at72. Fig. 2d presented the DLS-based size distributions of the final pro-ducts of ZrO2/AB dispersions with different ZPP contents. The ZrO2

nanoparticles exhibited particle sizes in the range of 1–5 nm at differentZPP contents (Table S1). The content of ZPP used in the precursor hadno significant influence on the sizes of particles, and in-situ formation ofultrasmall ZrO2 nanoparticles with narrow size distributions was per-formed in the RPB reactor.

Due to the formation of ZrO2 nanoparticles in the silicone networkby the RPB method, a large area of network structure can be found inthe TEM (Fig. 3a and b), which is mainly due to the high contrast ofzirconia as a metal oxide. However, no particles were observed frompure silicon resin in the TEM images (Fig. S3), because the commercial

Fig. 5. (a) Transmittance and (b) haze of the cured AB and ZrO2/AB films. (c) The transmission after ultraviolet irradiation of the cured AB and ZrO2/AB films. (d)The transmission after heat treatment of the cured AB and ZrO2/AB films. (e) RI of the cured AB and ZrO2/AB films. (f) The remaining weight ratio of the cured ABand ZrO2/AB resins.

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pure silicon resin is oligomer and the contrast of silicon resin under theTEM was low. In order to verify the formation of the network structure,we conducted several control experiments, which were RPB approachwithout adding ZPP, UR approach with adding ZPP, and UR approachwithout adding ZPP (Table S2). According to the analysis of Figs. S4–6,the combination of zirconium oxide formed by hydrolysis of ZPP andsilicone was the main factor for forming a network structure, and theinfluence of factors such as high-gravity condition, acidification andsolvent modification can be excluded. Significant lattice structures wereobserved from the HRTEM in Fig. 3c and d. Therefore, we hypothesizedand analyzed that these lattices were amorphous or newly crystallizedzirconia nanoparticles attached to the original silicone network struc-ture. Fig. 3e presents the schematic structure of the hybrid nano-zirconia/silicone estimated based on the experimental observations.

Fig. 4a and b shows the FTIR spectra of ZrO2/silicone different ZPPcontents. The peaks at 2940 cm−1 and 700 cm−1 were considered to bethe characteristic peak of methyl groups (-CH3) [35,36]. The peaks at1662 cm−1 and 1752 cm−1 were the characteristic peaks of phenylgroups and vinyl groups, which indicated that when the ratio of A:B kitis 1:1, the vinyl groups in blank samples did not react completely [35].However, with the addition of ZPP, the vinyl groups disappeared. Theseresults demonstrate that the addition of ZPP may result in somestructure (e.g. Si–O–Zr) affecting the formation of the silicone network.The peaks at 1020-1150 cm−1 in FTIR spectra of samples represent theSi–O–Si network [36]. In previous reports, ZPP has been used as a rawmaterial to synthesize transparent zirconium-phenyl siloxane hybridand used it in the field of LEDs encapsulation [22]. The zirconium-phenyl siloxane hybrid exhibited superior performances with a highoptical transparency and exhibited high thermal stability, compared tothe commercial LEDs encapsulation. Therefore, it is speculated that ZPPnot only reacts with water to form zirconia, but also a small portion ofZPP is directly bonded to the silica structure. As can be seen from thecurves in Fig. 4c and d, with the increase of ZPP content in the resinfrom 0 to 20 wt%, the weight loss ratio decreased from 56% to 48%,and the strongest exothermic peak shifted from 650 °C to 700 °C. This

proves that adding ZPP helps to improve the heat resistance of thecomposite LED encapsulation adhesive.

Fig. 5a shows the transmittance of the hybrid films after normalheating and curing. The transmittance of the films was higher than 90%in the visible region and the samples with ZPP addition show a certaindecrease in transparency in the ultraviolet region, which was due to theabsorption of ultraviolet light by zirconia. The haze value of the na-nocomposite films was all below 4, and there is no inevitable re-lationship between the haze results and the content of ZPP (Fig. 5b).Fig. 5c shows the transmittance after ultraviolet aging by 365 W UVlamp for 24 h. It can be found that the transmittance of the blanksample is strongly absorbed in the ultraviolet region, which is mainlybecause there are still unreacted double-bond groups in the nano-composite material. The transparency of ZPP samples after UV agingwas not significantly decreased compared with that before. Fig. 5dshows that the hybrid ZrO2/silicone exhibited lower transparency thanthe previous films after heating at 150 °C for 48 h. The RI of the pure ABresin and the ZrO2/AB hybrid materials was determined by an ellipseleaning meter in the wavelength range from 300 nm to 800 nm. Fig. 5eshows that the addition of ZPP can significantly improve the RI of theencapsulating material. The value was increased to 1.63 for the ZPP/AB20 with the highest ZPP content of 20 wt%. The specific step of thewater absorption experiment is to first take the films and soak it inwater at room temperature. After 24 h, remove the excess water fromthe surface and the bottom of the slide with the lens paper, and measurethe weight (Wwet). Then put it in a vacuum oven and dry it under va-cuum at 120 °C for 24 h to measure the weight (Wdry). The water uptakecan be expressed by the following formula.

= ×W W

WWater uptake (%) 100%wet dry

dry

Then 100 minus the water uptake can be used as an indicator of thewater absorption resistance of the nanocomposite resins. It can be seenfrom Fig. 5f that the remaining weight ratio of samples are closed to100%, while the water absorption rate of composite samples added

Fig. 6. (a) The emission spectra and (b) CCT valuesof LEDs devices fabricated by using ZrO2/siliconeencapsulation materials with different contents ofZPP (0, 1, 10, 20 wt%) at current of 100 mA. Theemission spectra of LEDs devices at various currents(10, 20, 40, 60, 100 mA) fabricated by using ZrO2/silicone encapsulation materials with (c) 0 wt% and(d) 10 wt% contents of ZPP.

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with ZPP remains at a low level.The surface morphology of the zirconia/silicone hybrid films was

then observed by SEM as shown in Fig. S7. By observing the mor-phology of the films with different proportions of ZPP, the agglom-eration point was reduced compared with the previous studies [37],which proved that the RPB method could effectively reduce the ag-glomeration phenomenon in the process of strengthening the molecularmixing. In addition, comparing the group of 1 wt% to 20 wt% films, it

can be observed that the increase of ZPP additive amount leads to moreclusters. Furthermore, the addition of ZrO2 nanoparticles to the en-capsulation materials of white LED devices is promising for improve-ment of the light output and correlated color temperature (CCT) forhealthy lighting [16]. The scattering effect of the ZrO2 nanoparticles inthe resin prevented the original blue light from escaping the resin di-rectly, which increased the possibility to excite the yellow phosphor[38]. Therefore, the increased utilization rate of the blue light increased

Fig. 7. The luminous flux (a), light output power (c) and luminous efficiency (d) of LEDs devices fabricated by using ZrO2/silicone encapsulation materials withdifferent contents of ZPP at various currents. (b) The luminous flux values of LEDs devices fabricated by using ZrO2/silicone encapsulation materials with differentcontents of ZPP at 100 mA. (e, g) CIE color coordinates of the white LEDs devices. (f) The optical photograph of the encapsulated white LEDs at lighting state.

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the output of the yellow light, resulting in the enhancement of lumenefficiency. In this work, we prepared white LED devices by using ournanozirconia/silicone nanocomposite encapsulation material andcommercial YAG phosphors. Fig. 6a shows the emission spectra of LEDsdevices fabricated by ZrO2/silicone with ZPP contents of 0, 1, 10, and20 wt%. As the amount of ZPP increases, the proportion of blue lightdecreases and the proportion of yellow light increases. The proportionvalues of blue light in the RGB color ratio of 0, 1, 5, 10, 15, 20 wt%were 3.1, 1.9, 1.8, 1.8, 1.7, 1.3, respectively. Fig. 6b shows the CCTwith the different concentrations of ZPP devices. The CCT of the re-ference sample without ZrO2 nanoparticles was 5413 K, and the valueof CCT decreased as the concentration of ZPP increased because of thehigher yellow conversion ratio. Fig. 6c and d show the emission spectraof LEDs devices at different currents, which were fabricated by hybridZrO2/silicone encapsulation materials with ZPP contents of 0 wt% and10 wt%. It was noted that the currents did not influence the proportionof light in the emission spectra.

The luminous flux, the luminous efficiency, and the light outputpower measured using a calibrated integrating sphere are plotted inFig. 7a–d as a function of injection currents ranging from 10 to 100 mA.The luminous flux of our LEDs devices encapsulated with pure AB si-licone resin was 13.28. With the addition of ZrO2 nanoparticles in thesilicone for encapsulation, the luminous flux of the LEDs was enhanced,which was attributed to two reasons. One reason is that the refractiveindex value of the encapsulating material rises. The other reason is thatas the amount of ZrO2 nanoparticles increases, the scattering phe-nomenon is more significant, and the proportion of blue light convertedinto yellow ray increases. The value of luminous flux is a measure of thevisible light energy from the light source that must be perceived by thehuman eye, but the visual sensitivity of the human eye to light of dif-ferent wavelengths is different. Studies have shown that the human eyeis most sensitive to green light with a wavelength of 555 nm, and thesensitivity of the human eye to brightness can be described by a relativesensitivity function curve (Fig. S8). Therefore, the amount of blue lightis reduced, and the increase in the amount of yellow light can greatlyincrease the lumen value of the device. A maximum in luminous fluxwas observed with a value of 31.34 for the LEDs encapsulated with 5 wt% of ZPP, which was increased by 236% compared with the controldevices under 100 mA electric current (Fig. 7b). It was noted that al-though the hybrid encapsulation films with 10 wt%, 15 wt%, and 20 wt% of ZPP exhibited higher refractive index values than the film with5 wt%, the light extraction efficiency of the devices did not increase.When the ZPP content continues to increase, although the RI of theencapsulant was increased, and the stay time of the blue light in theadhesive layer was prolonged because of the scattering effect of ZrO2

nanoparticles. In principle, the chances of blue light turning to yellowlight will increase and the blue light will decrease. Nevertheless, thechance of light conversion into heat also increases and the efficiency ofthe overall light extraction may decrease. The color quality of devicesalso be evaluated by a widely adapted standard, which is to measure itschromaticity coordinates under normal operations. The chromaticitycoordinates of the samples with different contents of ZPP of 10–100 mAare shown in Fig. 7e. Fig. 7f shows the actual luminescent pictures ofLEDs devices with different amounts of additives (a simple battery testusing two 1.5 V dry cells with a total voltage of 3 V). As the ZPP contentincreased, the chromaticity coordinates gradually shied to the yellowregion, and this observation indicated that the intensity of yellow lightbecame stronger and finally led to lower CCTs. Fig. 7g illustrates thedetailed shifting of chromaticity coordinates with different contents ofZPP dopants at current injections of 10–100 mA. As the current in-creases, the X and Y values of the chromaticity coordinates decrease.These experimental results demonstrated that high-gravity-hydrolysisapproach enables facile preparation of transparent nanozirconia/sili-cone encapsulation materials of LEDs devices with low blue light ratioand CCT, which can be considered to fall under the banner of healthylighting.

4. Conclusion

In summary, we develop a scalable high-gravity-hydrolysis ap-proach for synthesis of nanozirconia/silicone composites for LEDs en-capsulation. The method of strengthening micro-mixing by RPB reactorcan effectively reduce the particle size and degree of agglomeration ofZrO2 nanoparticles. Compared to the devices encapsulated by pure si-licone, the use of ZrO2/silicone hybrid encapsulation benefited the lightextraction efficiency of LEDs devices due to the enhanced refractiveindex and adjusted the ratio of blue to yellow light. The proportion ofblue light in the RGB color ratio of the composite package with ZrO2

addition decreased from 3.1% to 1.3% compared with those LEDs en-capsulated by commercial silicone. This study presents an effectivemethod for hybrid encapsulation in moving toward their commercia-lization and advanced applications in optical devices for saving energyand healthy lighting.

Acknowledgments

We are grateful for financial support from National Key Researchand Development Program of China (2017YFB0404302/2017YFB0404300), the National Natural Science Foundation of China(21808009, 21622601) and the Beijing Natural Science Foundation(2182051).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.nanoen.2019.05.024.

Declarations of interest

The authors declare that they have no conflict of interest.

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Xianglei He is currently a Ph.D. candidate under the su-pervision of Prof. Jian-Feng Chen at the State KeyLaboratory of Organic Inorganic Composites in BeijingUniversity of Chemical Technology, China. He receivedBachelor's degree (2016) in Beijing University of ChemicalTechnology in China. His current research focuses on High-gravity-assisted scalable synthesis of functional nanoma-terials for high performance LEDs devices.

Ruijie Tang is currently a Master candidate under the su-pervision of Prof. Yuan Pu at the Research Center of theMinistry of Education for High Gravity Engineering andTechnology in Beijing University of Chemical Technology,China. She received Bachelor's degree (2018) in BeijingUniversity of Chemical Technology in China. Her currentresearch focuses on functional nanomaterials for high per-formance LEDs devices.

Yuan Pu is currently a professor at the Research Center ofthe Ministry of Education for High Gravity Engineering andTechnology in Beijing University of Chemical Technology,Beijing, China. She received a Ph.D. degree (2010) in che-mical engineering under the supervision of Prof. Jian-FengChen from Beijing University of Chemical Technology,China. Her research interests focus on chemical en-gineering, nanomaterials and devices.

Jie-Xin Wang is currently a professor at the ResearchCenter of the Ministry of Education for High GravityEngineering and Technology and the State Key Laboratoryof Organic Inorganic Composites in Beijing University ofChemical Technology, Beijing, China. He received a B.E.degree and a Ph.D. degree in chemical engineering fromBeijing University of Chemical Technology, China, in 2001and 2006, respectively. His research interests focus on highgravity technology, nanomaterials and nanodrug deliverysystems.

Zhi Wang is currently an undergraduate student at theCollege of Materials Science and Engineering in the BeijingUniversity of Chemical Technology, China. His current re-search focuses on functional nanomaterials for high per-formance LEDs devices.

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DanWang is currently a professor at Research Center of theMinistry of Education for High Gravity Engineering andTechnology and State Key Laboratory of Organic InorganicComposites in Beijing University of Chemical Technology,China. Dr. Wang received a B.E. degree in materials scienceand engineering and a Ph.D. degree in optical engineeringfrom Zhejiang University, Hangzhou, China, in 2008 and2013, respectively. He was a visiting scholar at HannamUniversity in Korea from 2008 to 2009 and at Case WesternReserve University in USA from 2013 to 2015. His researchinterests focus on optical nanomaterials and process in-tensification.

Jian-Feng Chen is the Member of the Chinese Academy ofEngineering and has been promoted to Secretary-general ofthe Chinese Academy of Engineering since June 2018. He isalso Director of State Key Laboratory of Organic-InorganicComposites, Director of Research Center of the Ministry ofEducation for High Gravity Engineering and Technology inBeijing University of Chemical Technology, China. He re-ceived a BSc degree and a PhD degree from ZhejiangUniversity in Chemical Engineering in 1986 and 1992, re-spectively. His research interests involve process in-tensification (especially high gravity technology), nano-materials and nanodrug delivery systems.

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