Transparent Dispersions of Monodispersed ZnO Nanoparticles with

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Transparent Dispersions of Monodispersed ZnO Nanoparticles with Ultrahigh Content and Stability for Polymer Nanocomposite Film with Excellent Optical Properties Xie-Jun Huang,‡,§ Xiao-Fei Zeng,*,‡,§ Jie-Xin Wang,*,†,‡,§ and Jian-Feng Chen†,‡,§ †

Beijing Advanced Innovation Center for Soft Matter Science and Engineering, ‡State Key Laboratory of Organic−Inorganic Composites, and §Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, PR China ABSTRACT: A novel route was proposed for the synthesis of transparent dispersions of monodispersed zinc oxide (ZnO) nanoparticles via a high-gravity reactive precipitation combined with “inorganic−organic successive layer coating” method. The as-prepared nanodispersion had a uniform particle size of about 4 nm, high stability of over 12 months, and still remained transparent at ultrahigh solid content of 60 wt %. Highly transparent polylactide (PLA)/ZnO and PLA/ZnO/ cesium doped tungsten oxide CsxWO3 nanocomposite films were further fabricated by a simple solution mixing method. Because of the uniform dispersion of nanoparticles, PLA/ZnO films could maintain the same visible transmittance as pure PLA film even when ZnO content reached 60 parts per hundred of PLA resin by weight (phr). Furthermore, the films could shield almost 100% ultraviolet radiation at ZnO content of up to 5 phr, thereby realizing good antiaging property. Besides their excellent transparency and UV-shielding property, PLA/ ZnO/CsxWO3 films could also block 90% near-infrared radiation, thereby displaying excellent heat insulation ability. It could be envisioned that this nanocomposite would have great potential in many applications such as smart windows, agricultural film, fabrics, etc.

1. INTRODUCTION Organic−inorganic nanocomposite materials exhibit remarkable enhanced properties and have been heavily investigated during the past decades.1,2 The combination of unique properties of both components opens a new path to novel materials. However, the bottleneck for the development of organic− inorganic nanocomposites still remains, which focuses on lowcost and large-scale preparation of nanoparticles (NPs) as well as their good dispersion and high content in nanocomposites with preferred properties.3,4 Poor dispersion of NPs in the matrix, mainly due to particle aggregation, will lead to fragility of composites along with performance reduction. In particular, better NPs dispersity or even monodispersity is required for optically transparent polymer-based functional nanocomposites, which are widely applied in many kinds of fields such as flexible displays, smart windows, solar cells, and electronics as light-emitting diodes or thin-film transistors.5−9 Presently, some transparent functional nanocomposites have been achieved by incorporating various NPs10−12 with good dispersity into optical polymers. Among them, transparent nanocomposites based on ZnO NPs, have attracted the most attention owing to their high heat capacity, low thermal expansion, excellent photocatalysis, nontoxicity, and effective UV blocking properties.13−17 However, there are still several challenges. First, the transparency of optical polymer/ZnO © XXXX American Chemical Society

nanocomposites will decrease with the increase of the solid content of ZnO in composites. This is because it is difficult to achieve good dispersity and high content of ZnO NPs in nanodispersions as key functional additives.18−20 Second, ZnObased nanocomposites probably suffer from an accelerated decomposition due to the photocatalysis of ZnO NPs when exposed to UV radiation.21,22 This issue is scarcely focused and needs to be urgently resolved. Third, optical ZnO-based nanocomposites with only a single function of UV blocking are hard to meet the requirements of practical applications. Mutifunctional transparent nanocomposites are required. However, the introduction of other functional NPs besides ZnO into optical polymer is an obstacle owing to more complicated interaction behaviors of different NPs and polymer. Therefore, it is of great significance to develop a novel route for the efficient preparation of dispersions of welldispersed ZnO NPs with high content and low photocatalysis and fabricate high-performance and multifunctional ZnO-based transparent nanocomposites. Received: Revised: Accepted: Published: A

November 24, 2017 March 2, 2018 March 5, 2018 March 5, 2018 DOI: 10.1021/acs.iecr.7b04878 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Rotating packed bed (RPB) has recently been applied as an ideal reactor for the preparation of inorganic NPs with smaller size owing to the greatly intensified micromixing from the generated high-gravity environment.23−26 Compared with a traditional stirred tank reactor, RPB is considered to be a promising platform for massive production of nanomaterials and impresses us with its outstanding advantages in preparing NPs such as shorter processing time, smaller size, and narrower size distribution.27 To the best of our knowledge, little research has been done to adopt the RPB for the preparation of transparent dispersions of monodisperse ZnO NPs owing to the higher level of difficulty than nanopowders. Herein, we developed a successive synthesis route to obtain highly transparent dichloromethane (DCM) dispersions of monodispersed ZnO NPs with high content and stability based on high gravity reactive precipitation combined with an “inorganic−organic successive layer coating” (IOSLC) method. The as-prepared ZnO NPs had a very thin inorganic SiO2 coating layer to weaken the photocatalysis of ZnO and organic molecule layer to avoid aggregation. Furthermore, transparent degradable polylactic acid (PLA)/ZnO nanocomposite films with single UV blocking function were facilely fabricated via a simple solution mixing method. Subsequently, PLA/ZnO/ CsxWO3 (CWO) films with multifunctions of UV blocking and near-infrared radiation (NIR, wavelength ranging from 780 to 2500 nm) light insulation were successfully achieved by simultaneous introduction of NPs of ZnO and CWO (a kind of NIR absorbing nanomaterial28−32) with close polarity and the same surface charge into PLA. The related optical properties and thermal insulation ability were also investigated.

Figure 1. Process flow diagram (PFD) of the preparation process of ZnO nanodispersion: (a) Zn(Ac)2 solution (preheated); (b) KOH solution; (c) pump; (d) flowmeter; (e) RPB reactor; (f) modification tank.

SiO2-coating process was completed, the mixture of silane coupling agent KH570 and OTS (KH570/OST = 100 wt %, KH570/ZnO = 25 wt %) was dropwise added into the tank and further stirred for another 30 min. The reaction was then ended by the addition of deionized water. After being left overnight, the product was filtered and washed sequentially with deionized water and ethanol three times. The obtained filter cake was dispersed in DCM, followed by rotary evaporation under 40 °C to remove ethanol. After each evaporation, DCM was added to disperse the product again. The rotary evaporation process was repeated three times. Finally, transparent DCM dispersions of monodispersed ZnO NPs were achieved, denoted as T-ZnO (surface treated ZnO NPs with inorganic and organic coating layers). For comparisons, a blank untreated ZnO (UT-ZnO) sample without any surface coating and an unmodified ZnO (UM-ZnO) sample with only an SiO2 coating layer were prepared under the same processing conditions. 2.3. Preparation of PLA/ZnO Nanocomposites. The PLA/ZnO nanocomposites were fabricated by a solution mixing method. A certain amount of PLA was first dissolved in DCM at a room temperature with vigorous stirring for 2 h to form a homogeneous 5 wt % PLA solution. Then the asobtained transparent DCM dispersion of monodispersed ZnO NPs was dropwise added into the above solution with stirring for 2 h. The formed transparent mixture was cast into a glass mold and placed at ambient temperature for 2 h. Finally, the film was dried at 30 °C for another 12 h in vacuum to obtain PLA/T-ZnO nanocomposite film with a thickness of 0.05 mm. In addition, UT-ZnO was also used to prepare PLA/UT-ZnO nanocomposites with the same method. 2.4. Preparation of PLA/ZnO/CWO Nanocomposites. The preparation steps of PLA/ZnO/CWO nanocomposites were the same as for PLA/ZnO nanocomposites except that the ZnO nanodispersion was replaced with a ZnO/CWO nanodispersion. The ZnO/CWO nanodispersion was typically prepared as follows. Briefly, CWO dispersed in BA was first transferred to DCM by a rotary evaporation at 50 °C and dried at 60 °C in vacuo for 12 h to remove BA completely. Afterward, the ZnO DCM dispersion was added to the above CWO DCM dispersion and further stirred for 1 h to obtain a transparent ZnO/B-CWO (CWO from BA nanodispersion of CWO) DCM nanodispersion. For comparison, commercial CWO DCM nanodispersion (D-CWO) was directly mixed with the ZnO DCM nanodispersion to obtain the ZnO/D-CWO DCM dispersion. 2.5. Characterization. The crystal structure of ZnO NPs was characterized by X-ray diffraction (XRD) with CuK α radiation (λ = 0.15406 nm) on a SHIMADZU 6000 X-ray diffractometer. The zeta potential measurement was performed

2. EXPERIMENTAL SECTION 2.1. Materials and Setup. Zinc acetate dihydrate (Zn(Ac)2·2H2O), potassium hydroxide (KOH), methanol, ethanol, dichloromethane, and tetraethoxysilane (TEOS) were purchased from Beijing Tong Guang Fine Chemical Co., Ltd. (China). PLA was supplied by Teijin Chemicals, Ltd. γMethacryloxypropyltrimethoxysilane (KH570) and octyltrimethoxysilane (OTS) were separately obtained from Alfa Aesar Chemical Co., Ltd. (China) and Shanghai Vulcan Chemical Reagent Co., Ltd. (China). All of the chemicals were of analytical purity and used without further treatment. Dispersions of CWO NPs in butyl acetate (BA) and DCM were both obtained from Nanomaterials Technology Pte. Ltd. (Singapore). Deionized water was obtained by a Hitech Laboratory Water Purification System DW100 (Shanghai Hitech Instruments Co., Ltd.) and used throughout the experiments. The key reactor for preparing NPs was a rotating packed bed (RPB) apparatus, mainly consisting of a wire-mesh packed rotator, a fixed casing, two liquid inlets, and an outlet. More detailed information on RPB could be found in our previous works.27,33 2.2. Preparation of Transparent ZnO Nanodispersions. The experimental equipment for the preparation of transparent ZnO nanodispersion is schematically shown in Figure 1. First, 0.34 mol/L Zn(Ac)2·2H2O methanol solution (60 °C) and 0.6 mol/L KOH methanol solution (20 °C) were prepared. The two solutions were simultaneously pumped into the preheated (70 °C) RPB with both flow rates of 200 mL/ min, and the rotating speed of RPB was set at 1800 rpm. Afterward, ZnO nanosuspension was collected from the outlet and introduced into a stirring tank. TEOS (TEOS/ZnO = 15 wt %) was immediately added and reacted for 30 min. After the B

DOI: 10.1021/acs.iecr.7b04878 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

After the storage for 2 months, the nanodispersion still maintained good transparency. In contrast, the UT-ZnO suspension had a severe agglomeration (Figure 2b). As a result, the suspension was opaque, and particles sedimentated after 2 days. UV−vis transmittance spectra indicated that the asprepared nanodispersion exhibited a high transmittance of 98% at 550 nm and could block almost 100% UV ray under 357 nm at a low ZnO content of only 1 wt %. Moreover, the nanodispersion still kept a transmittance of 88% at 550 nm and had the UV shielding ceiling of 382 nm when its solid content reached as high as 60 wt %. Figure 3 gives XRD patterns and FT-IR spectra of UT-ZnO and T-ZnO, as well as TGA curves of UM-ZnO and T-ZnO. XRD results indicated that all peaks of both samples were indexed, corresponding to hexagonal phase (JCPDS No. 361451). However, the peaks of T-ZnO were slightly broadened and weakened, especially in the (100), (101), and

by MALVERN Zetasizer Nano ZS90. The morphology and the dispersity of ZnO NPs in nanodispersion and PLA/ZnO nanocomposites were obatined with a Hitachi HT7700 EXLENS transmission electron microscope (TEM). FTIR spectra were obtained with a Bruker Vector 70 spectrometer employing a KBr pellet method. The thermal behavior was examined on a Netsch STA449C thermal gravimetric analyzer at a heating rate of 10 °C·min−1 under air. The optical properties were characterized by a SHIMADZU UV-2501 UV− vis spectrometer in the range of 300−800 nm and PerkinElmer lambda 950 UV−vis spectrometer in the range of 300−2500 nm, respectively. The antiaging accelerated test was processed in a radiation resistance testing machine bought from Wuxi Silian Science Technology Co., Ltd. The insulation tester was supplied by Shenzhen Linshang Technology Co., Ltd. to detect the heat shielding properties of nanocomposite films.

3. RESULTS AND DISCUSSION Figure 2 shows typical TEM images and the visual photographs of the T-ZnO nanodispersion and UT-ZnO suspension as well as UV−vis transmittance spectra and photographs of ZnO nanodispersions with various solid contents. Clearly, T-ZnO NPs were nearly monodispersed and had a spherical morphology, uniform particle size of about 4 nm (Figure 2a).

Figure 2. TEM images of T-ZnO DCM nanodispersion (a) and UTZnO DCM suspension (b); photographs of T-ZnO nanodispersion (c) and UT-ZnO suspension (d) (a solid content of 10 wt %); UV−vis transmittance spectra and photographs (e)of T-ZnO nanodispersions with various solid contents.

Figure 3. (a) XRD patterns of UT-ZnO (1) and T-ZnO (2); (b) FTIR spectra of T-ZnO (1), UT-ZnO (2), SiO2 (3), OST (4), and KH570 (5); (c) TGA curves of UM-ZnO (1) and T-ZnO (2). C

DOI: 10.1021/acs.iecr.7b04878 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 4. UV−vis transmittance spectra of PLA/T-ZnO (a) and PLA/UT-ZnO (b); photographs of PLA/T-ZnO (c) and PLA/UT-ZnO films (d) with various contents of NPs from 0 to 60 phr; TEM images of PLA/T-ZnO (e) (10 phr) and PLA/UT-ZnO (f) (10 phr) nanocomposites.

(002) lattice planes, indicating that T-ZnO NPs had lower crystallinity due to the coated organic and inorganic layers. No peak concerning SiO2 was observed mainly due to the formation of very thin amorphous SiO2 coating layer. The corresponding FTIR spectra exhibited there were remarkable strong peaks at 480 and 3400 cm−1, assigned to Zn−O bond and O−H stretching vibration bond, respectively. By further comparing curves (1) with (3−5), it could be found that the characteristic broad peak at ∼1110 cm−1 corresponded to Si− O−Si stretching vibration could be owing to the presence of SiO2 coating layer on the surface of T-ZnO. The characteristic peaks at 1720 and 1640 cm−1 belonging to CO and CC stretching bonds and the strong absorption peaks at 2850− 2960 cm−1 attributed to C−H vibration confirmed the successful modification of KH570 and OST. TGA results indicated that T-ZnO sample had an obvious weight loss of about 13.4 wt % in the temperature ranging from 250 to 450 °C, mainly due to the decomposition of organic molecules

coated on the surface of particles, as compared to UM-ZnO. In addition, the weight loss of UM-ZnO possibly resulted from the elimination of −OH on the surface of NPs. Figure 4 shows UV−vis transmittance spectra, visual photographs, and TEM images of PLA/T-ZnO and PLA/ UT-ZnO nanocomposite films. Clearly, PLA/T-ZnO nanocomposite films were highly transparent and had a transmittance close to pure PLA film. With the increase of ZnO content from 0 to 60 parts per hundred of PLA resin by weight (phr), the transparency of the film had no obvious decrease, still keeping the transmittance of 97% at 550 nm even at 60 phr. This completely indicated the excellent dispersity of the asprepared T-ZnO in PLA at an ultrahigh solid content. More importantly, the 100% UV radiation screening ceiling of PLA/ T-ZnO films had a redshift from 345 to 355 nm with the increased NPs content from 5 phr to 60 phr, beneficial to better blocking of UV ray. In addition, the transparent nanocomposite film could emit yellow light under UV irradiation (365 nm) due D

DOI: 10.1021/acs.iecr.7b04878 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research to the photoluminescence effect of ZnO.34 In contrast, PLA/ UT-ZnO films appeared translucent and then opaque with the increased UT-ZnO content. This distinct difference in visual transparency was mainly ascribed to much better dispersity of T-ZnO in the composite films (Figure 4e). Infrared spectrometry is an effective tool for studying the aging of polymers.35 Figure 5 shows the FTIR spectra of PLA

Figure 5. FT-IR spectra of PLA and PLA/T-ZnO nanocomposite films before and after UV irradiation for 72 h.

and PLA/T-ZnO (5 phr) nanocomposite films before and after an aging procedure of 72 h UV irradiation. For pure PLA film, the characteristic band at ∼1630 cm−1 ascribed to CC stretching vibration was greatly intensified due to the break of C−H bond after UV irradiation for 72 h. Furthermore, the characteristic band at ∼3500 cm−1 belonging to O−H stretching vibration was significantly increased mainly because of the fracture of PLA polymer chains, indicating the aging effect on polymer.36 Correspondingly, for PLA/T-ZnO film, the intensification of the band at 3500 cm−1 was greatly reduced, while the intensity of the band at ∼1630 cm−1 almost kept unchanged. This is because T-ZnO NPs played a UV absorbing role during the UV exposure time, and brought an obviously decreased catalytic degradation effect on nanocomposites owing to the protection of SiO2 layer. Figure 6 presents photographs of ZnO/D-CWO and ZnO/ B-CWO DCM nanodispersions before and after the storage of a week, UV−vis-NIR spectra of D-CWO and B-CWO DCM nanodispersions before and after mixing with ZnO DCM nanodispersion, as well as pure PLA film and PLA/ZnO/BCWO nanocomposite film. Obviously, ZnO/B-CWO DCM nanodispersion had no agglomeration and still maintained a good transparency, indicating that B-CWO NPs had a better compatibility with ZnO. However, the counterpart had first a decreased transparency and then obviously sedimentated after the storage of a week, as shown in Figure 6a,b. This is mainly attributed to the close polarity and the same negative surface charge of B-CWO DCM nanodispersion (zeta potential = −25.4 mV) and ZnO DCM nanodispersion (zeta potential = −40 mV). UV−vis-NIR results indicated that both CWO nanodispersions exhibited similar excellent NIR light shielding ability and good visible light transmittance. However, after mixing with ZnO, ZnO/D-CWO nanodispersion had a big reduction in visible light transparency from 74% to 24% at 500

Figure 6. Photographs (a) of ZnO/D-CWO (1) and ZnO/B-CWO (2) DCM nanodispersions before and after storage for a week; UV− vis−-NIR spectra (b) of D-CWO and B-CWO DCM nanodispersions before and after mixing with ZnO DCM nanodispersion, as well as pure PLA film, PLA/ZnO/B-CWO nanocomposite film (ZnO and CWO NPs with a same content of 5 phr) and the corresponding photograph (c).

nm due to the opposite surface charge of D-CWO (zeta potential = 29 mV). Simultaneously, ZnO/B-CWO nanodispersion kept almost unchanged transmittance and revealed a better UV shielding effect than B-CWO dispersion owing to the introduction of T-ZnO (the inset of Figure 6c). Furthermore, the obtained PLA/ZnO/B-CWO nanocomposite film exhibited excellent optical properties, including high transparency and blocking of more than 90% NIR at 1000 nm and almost 100% UV under 345 nm (Figure 6c). To better evaluate heat insulation properties of nanocomposite films, a comparative study of thermoregulation effect was conducted. Figure 7a shows the structure illustration E

DOI: 10.1021/acs.iecr.7b04878 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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change of temperature with irradiation time and the corresponding visual photographs of three glass samples are given in Figure 7d. It could be obviously seen that after continuous NIR irradiation for 1 min the temperature of the area under the ordinary glass had a noticeable increase of 11.2 °C from 29 to 40.2 °C and still kept an increase trend. Correspondingly, the temperature of the area under the glass coated with PLA/ZnO/B-CWO nanocomposite film had an only small increase of 2.3 °C from 29 to 31.3 °C, suggesting that a significant amount of irradiation was blocked by nanocomposite film. At this time, the temperature of the glass increased obviously owing to the photothermal effect of CWO.37 However, the temperature of the area under PLA film coated glass had an increase of 10.2 °C from 29 to 40.2 °C, nearly close to that under ordinary glass, indicating a very limited heat-insulation effect of PLA. Based on the above results, it could be inferred that PLA/ZnO/B-CWO nanocomposite film was quite effective for heat insulation and, therefore, very promising in practical applications like smart windows for buildings and vehicles.

4. CONCLUSIONS In summary, highly transparent dispersions with ZnO NPs monodispersed in DCM were synthesized by using high gravity precipitation in a RPB reactor combined with an “IOSLC” method. The as-prepared ZnO NPs had a mean size of 4 nm, a high content of 60 wt %, and high stability for more than 12 months. Highly transparent PLA/ZnO and PLA/ZnO/CWO nanocomposite films were further fabricated via a solution mixing method. PLA/ZnO nanocomposite films maintained the visible transmittance as pure PLA film even at 60 phr content of NPs and showed good antiaging property. The PLA/ZnO/CWO nanocomposite film with both 5 phr content of ZnO and CWO could block almost 100% UV and 90% NIR ray. A simulated heat ray shielding test indicated the outstanding heat insulation ability of PLA/ZnO/CWO nanocomposite with a low temperature increase of 2.3 °C, compared to over 10 °C of the counterpart after NIR irradiation for 1 min. It could be envisioned that this environmental-friendly nanocomposite film with excellent UV and NIR irradiation shielding properties would be promising in many applications such as optical flexible devices requiring for high transparency, antiaging, and heat insulation abilities. In brief, this work develops a novel way to the synthesis of monodispersed NPs, and contributes to the fabrication of transparent multifunctional organic−inorganic nanocomposites, which will be widely used in the future.

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Figure 7. Schematic illustration of the heat insulation tester (a); photographs (b, c) of heat insulation tester equipped with ordinary glass (left) and PLA/ZnO/B-CWO nanocomposite film coated glass (right), ordinary glass (left) and PLA film coated glass (right); the temperature variation with irradiating time and visual photographs (d) of three glass samples.

AUTHOR INFORMATION

Corresponding Authors

*Tel: +86-10-64447274. Fax: +86-10-64423474. E-mail: [email protected]. *Tel: +86-10-64449453. Fax: +86-10-64444784. E-mail: [email protected].

of the machine for heat insulation test. The tested glass was placed in a middle position with a NIR lamp irradiation from the up side, and the temperature variation of the bottom area was investigated. First, ordinary glass and PLA/ZnO/B-CWO film coated glass were tested (Figure 7b). Second, to verify the influence of PLA polymer on heat shielding, PLA/ZnO/BCWO film was replaced with pure PLA film (Figure 7c). The

ORCID

Xiao-Fei Zeng: 0000-0001-9010-0088 Jie-Xin Wang: 0000-0003-0459-1621 Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acs.iecr.7b04878 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This work was financially supported by the National Key Research and Development Program of China (2016YFA0201701/2016YFA0201700) and the National Natural Science Foundation of China (21776016 and 21622601).

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DOI: 10.1021/acs.iecr.7b04878 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research homogeneous CsxWO3 nanorods with broad near-infra-red absorption. Nanoscale 2013, 5, 6469.

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DOI: 10.1021/acs.iecr.7b04878 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX