Supercritical Fluid Assisted Dispersion of Nano-Silica Encapsulated

Jun 26, 2018 - The experimental nanocomposite films also showed improved thermal and ... Brumberg, Diroll, Nedelcu, Sykes, Liu, Harvey, Wasielewski, ...
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Article Cite This: ACS Appl. Nano Mater. 2018, 1, 3186−3195

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Supercritical Fluid Assisted Dispersion of Nano-Silica Encapsulated CdS/ZnS Quantum Dots in Poly(ethylene-co-vinyl acetate) for Solar Harvesting Films Md Abdul Mumin, Kazi Farida Akhter, Olabode O. Oyeneye, William Z. Xu, and Paul A. Charpentier*

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Department of Chemical and Biochemical Engineering, The University of Western Ontario, London, Ontario, Canada N6A 5B9 ABSTRACT: Transparent polymer/inorganic nanocomposites are of great interest for solar harvesting films with quantum dots (QDs) which provide light tuning to the existing polymeric material. However, achieving a high level of QD dispersion without compromising the optical, thermal, and mechanical properties has been a key challenge. In this work, we explore a novel supercritical carbon dioxide (scCO2) synthesis method that utilizes scCO2 to enhance the homogeneous dispersion of encapsulated and functionalized QDs into ethylene vinyl acetate (EVA) copolymer. The core/ shell CdS/ZnS QDs were first encapsulated with silica to improve their photoluminescence and stability. The silica surface of the QDs then vinyl functionalized by the addition of vinyltrimethoxysilane (before or during polymerization). QDs dispersion in the EVA polymer network was characterized by SEM, TEM, and CLSM. The results demonstrated that scCO2 improved the dispersion of QDs within the polymer matrix with uniform dispersion being observed for a two-step process (functionalization before polymerization). FTIR confirmed the successful functionalization on the silica surface. The vinyl acetate content of the EVA copolymer and nanocomposite was 33.5% calculated using the TGA. The optical study results show that with a very low loading of QDs (∼1%) up to 80% UV bleaching was prevented while retaining more than 90% visible light transmission. The experimental nanocomposite films also showed improved thermal and mechanical properties, showing the utility of this approach for next-generation solar harvesting films. KEYWORDS: quantum dots, silica encapsulation, polymer nanocomposites, supercritical CO2, optical transparency, UV shielding, thermal property, mechanical performances

1. INTRODUCTION Polymer matrices reinforced with nanoscale inorganic materials have been studied extensively by combining the benefits of the organic polymers and the inorganic nanoparticles (NPs).1−3 These hybrid materials provide high performance new functional materials that find applications in many industrial fields.3,4 Poly(ethylene-co-vinyl acetate) (EVA) has been a widely used transparent solar film with applications including photovoltaic (PV) encapsulants, greenhouse coverings, laminated glass, etc.5−7 However, controlling light and heat selective properties and stability are the major challenges of the neat EVA films.8 Recently, semiconductor nanocrystals, also known as quantum dots (QDs), have attained intense interest for their light harvesting properties.6,9,10 The most intriguing feature about QDs is that their absorption, emission, and band gaps can be tuned by changing the type, size, and shape of the QDs.11,12 Despite the potential advantageous optical properties of QDs, their reactivity, aggregation tendency, uncontrollable sizes, inconsistent emission, and photobleaching have hindered recent progress in polymer nanocomposites.9,13 We previously reported that silane © 2018 American Chemical Society

functionalized CdS and CdS/ZnS QDs could be incorporated into a EVA copolymer by extrusion in a twin-screw extruder.6 However, the prolonged photostability of these functionalized QDs was limited, while coverage of the QD surface atoms by the organic ligands may cause steric hindrance during further processing.14,15 We have also examined another approach mesoporous silica encapsulation of CdS and CdS/ZnS QDs loaded into EVA copolymer by extrusion.9 A controlled thickness of the silica layer on the QDs showed an enhancement of quantum efficiency and stability of the QDs. The biocompatible, chemically inert, and optically transparent silica layer minimizes the photo-oxidation and photobleaching of the QD materials. Porous silica minimizes the aggregation tendency of QDs and nonradiative recombination centers formation during the photo-oxidation.9 The optical properties of the nanocomposite films were improved significantly using this approach.9 However, uniform distribution of silica NPs Received: March 11, 2018 Accepted: June 26, 2018 Published: June 26, 2018 3186

DOI: 10.1021/acsanm.8b00390 ACS Appl. Nano Mater. 2018, 1, 3186−3195

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ACS Applied Nano Materials

(i.e., affects glass transition temperature, Tg), (iii) reduces the viscosity and interfacial tension of polymers, (iv) enhances phase separation, (v) sorbs in the porous component of the NPs (silica shell on QDs) and increasing the mobility of NPs, and (vi) enhances mass transfer and reaction kinetics, leading the penetration of NPs into pores of the polymer network.25,27,29−31 ScCO2 not only works as a solvent and modifier but also is a cleaning and drying agent. It avoids tedious and costly solvent removal processes; this is crucial in the case of nanocomposite synthesis using organic solvents.25 Here, CO2 can be easily spread out by releasing pressure to ambient conditions. Recently, QDs/polymer-based nanocomposites synthesis using scCO2 has emerged and reported by different research groups,11,32,33 but the strong interactions, uniform distribution, and stability are still major challenges that play a key role for long-term applications of these nanocomposites. Herein, monodispersed and high quantum efficient CdS/ ZnS core/shell QDs were synthesized by a colloidal synthesis approach. The QDs were encapsulated by a controlled layer of silica using a water-in-oil or reverse microemulsion technique. Then the silica shell of the QDs was functionalized by a silane coupling agent containing vinyl functionality. Finally, a graf ting-f rom polymerization of both monomers ethylene and vinyl acetate on the functionalized QDs was carried out under scCO2. The functionalization and polymerization reactions were explored by both one-pot and two-step techniques. The novel nanocomposites were then characterized in terms of their morphology and QDs dispersion into the matrix. Finally, optical, thermal, and mechanical properties were evaluated.

into the polymer network and stability under prolonged photoexcitation without losing optical transparency are still supreme challenges. This drove to our investigating a new technique to homogeneous disperse and stabilize these NPs into composites for prolonged solar harvesting materials. In general, there are two preparative methods (in situ and ex situ) reported for polymer−inorganic nanocomposites.16 In situ synthesis provides a facile and one-step fabrication route where the NPs are nucleated from the corresponding precursors and are grown inside a polymer matrix during polymerization.17,18 For successful synthesis of nanocomposites, the interaction between inorganic NPs and organic polymer is very crucial. However, controlled tailoring of the NPs surface and their coupling to the polymer matrix is difficult during in situ synthesis, leading to poor control of the monodispersity, crystallinity, and properties of the final products.19 Moreover, the unreacted reactants and byproducts can have adverse effects on the properties of the nanocomposites, which is another major drawback of this method.16 Another important route is ex situ synthesis of polymer nanocomposites in which premade or functionalized inorganic NPs are blended with the polymers or polymerization occurs in the presence of these NPs for direct coupling to the polymer matrix.16,19 Recently, we have reported QDs-EVA nanocomposite films with improved optical and heat retention properties by using an extrusion approach which is a simple and convenient way suitable for large scale industrial application.6,8,9 However, due to the high surface energy of the QDs and low compatibility with polymer, it becomes difficult to maintain good dispersibility of QDs within the polymer matrix. This can lead to decrease of the toughness and stability of the prepared composites for long span applications of nanocomposite films and coatings.16,20 The “graf ting-from”, a surface-initiated polymerization approach, has been shown as a very promising and versatile method.21−23 Here, surface engineering minimizes the interfacial energy and increases the hydrophobicity, which with controlled polymerization on the modified NP surface enhances the chemical bonding, compatibility, and dispersibility of the NPs within the polymer matrix.19,20 There is no published report found on surface engineered QDs incorporated into EVA copolymer using graft polymerization, especially for applications in solar harvesting film applications. Therefore, it is anticipated that by functionalizing the silica shell of the QDs with a silane coupling agent followed by grafting-f rom polymerization of ethylene and vinyl acetate monomers, both the distribution and stability of the QDs as well as the optical, heat retention, and mechanical performances of the nanocomposites can be improved. As a feasible “green” alternative to toxic organic solvent, supercritical CO2 (scCO2) has emerged as a functional solvent which can facilitate both polymerization and material processing.24−26 ScCO2 has a fairly low critical point (Tc = 31.1 °C and Pc = 7.4 MPa) with other advantages, including CO2 is inexpensive, nonflammable, nontoxic, and environmentally benign.27 Under supercritical conditions, scCO2 molecules interact with functional groups of EVA (in this case the vinyl acetate groups of EVA) to help solubilize the polymer chains by Lewis acid−base interaction between basic functional groups of polymers and acidic sites of CO2.28 ScCO2 (i) facilitates dissolution and swelling of the polymer, (ii) disrupts the intermolecular interaction of polymers and increases the mobility and distance between polymer chains

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. CO2 (99.99%, instrumental grade with dip tube), ethylene (99.99%, polymer grade), and argon (99.9%, ultrahigh pure) were purchased from Praxair Canada Inc. The moisture and oxygen of ethylene were eliminated by passing the ethylene through a stainless-steel column packed with molecular sieves (5 Å) and 20% copper oxide supported on alumina (Millipore Sigma, Canada). Vinyl acetate (≥99%) containing 3−20 ppm hydroquinone as inhibitor was purchased from Millipore Sigma, Canada. Before the polymerization reaction, hydroquinone was removed by passing the monomer through a disposable prepacked inhibitor remover column purchased from Millipore Sigma, Canada. QD precursors including cadmium chloride (technical grade), sodium diethyldithiocarbamate trihydrate (ACS reagent grade), zinc diethyldithiocarbamate (98%); silica precursor tetraethyl orthosilicate (reagent grade); all solvents including trioctylamine (TOA) (≥99%) (Fluka), cyclohexane (anhydrous, 99.5%), chloroform (≥99.5%), 1-hexanol (reagent grade, 98%), heptane (>99%), ammonia solution (ACS reagent, 28−30%); and all other reagents including glacial acetic acid, H2O2 (ACS reagent, 30%), ethyl chloroformate (≥98%), trioctylphosphine (TOP) (97%), and vinyltrimethoxysilane (VTMS) were also purchased from Millipore Sigma, Canada, and used as received. 2.2. Synthesis of Silica Encapsulated CdS/ZnS QDs. A modified pyrolysis approach of the single-molecular precursors was used to synthesize the core/shell CdS/ZnS QDs.6 The encapsulation of CdS/ZnS QDs by mesoporous silica was carried out using a reverse microemulsion technique based on sol−gel chemistry and hydrophobic interaction mechanism.9 2.3. Preparation of DEPDC Initiator. Diethyl peroxydicarbonate (DEPDC), the initiator of EVA polymerization, was prepared under a controlled temperature following a previously reported method.34 The yield of the initiator formation was analyzed and calculated using an iodometric titration standard method and verified by NMR analysis. 3187

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Scheme 1. Schematic Illustration of (i) Preparation of DEPDC (Initiator), (ii) Functionalization of Silica Encapsulated CdS/ ZnS QDs Using Vinyltrimethoxisilane, (iii) Grafting-f rom Polymerization of EVA on Vinyl Functionalized Silica Encapsulated QDs under scCO2, and (iv) Schematic Diagram Showing Swelling and Dispersion of QDs in Polymer under Organic Solvent and scCO2

another syringe pump (Isco 260D). The polymerization reaction was carried out 50 °C under 27.5 MPa for 20 h. After the polymerization, CO2 gas was vented out carefully, leaving the nanocomposite products in the reactor. The viscous EVA nanocomposite was collected from the autoclave and subsequently dried at 40 °C overnight under vacuum. Scheme 1 shows the functionalization and polymerization to prepare EVA nanocomposites under scCO2. 2.6. Material Characterization. The morphology of CdS/ZnS QDs, silica encapsulated QDs, and EVA nanocomposites was investigated using a Phillips CM10 transmission electron microscopy (TEM) under an accelerate voltage of 80 kV. For NPs imaging, their dispersion in ethanol was drop-casted on a Formvar-coated copper grid. For nanocomposite films, a small piece of 250 μm film on a polyhyroxy-aromatic acrylic embedded resin support was thin sliced using an ultramicrotome and then placed on a Formvar-coated copper grid. The size distribution of QDs and silica encapsulated QDs was estimated from TEM images using ImageJ software. The morphology of silica encapsulated QDs and EVA nanocomposites was also studied using a focused ion beam scanning electron microscope (FIB-SEM) (ZEISS 1540 XB). The presence of QDs and silica elements in EVA nanocomposites was confirmed using the energy dispersive X-ray (EDX). Thin films (100, 250, and 500 μm) of the experimental EVA nanocomposites were prepared using a Spectra-Tech Universal Film Maker (UFM) coupled with a hydraulic press (Carver Inc.) and a temperature controller. A laser scanning confocal microscope (ZEISS LSM 5 Duo) with an argon laser (emission wavelength 488 nm) was used for fluorescence imaging the planes of the experimental EVA nanocomposite films using an objective of 20× magnification. Vinyl functionalization of silica encapsulated QDs and functionality presence in the EVA nanocomposites were examined from the

To avoid highly reactive decomposition, concentrated DEPDC in heptane was stored at −20 °C. 2.4. Functionalization of Silica Encapsulation of CdS/ZnS QDs. The vinyl functionalization of mesoporous silica encapsulated QDs was carried out based on modification of a previously reported method.35 Typically, 50 mg of mesoporous silica encapsulated CdS/ ZnS QDs was dispersed in 25 mL of toluene. 15 μL of Milli-Q deionized water was added and stirred vigorously for 1 h for maximizing the water dispersion through the porous silica structure and hydroxylation of the silica surface.35 Then, 0.77 mL of vinyltrimethoxysilane (VTMS), a silane coupling agent, was added, and the mixture was stirred for 12 h at ambient temperature. The product was washed with excess of 2-propanol and centrifuged for 3/4 times. Finally, the collected product was dispersed in toluene (2 mL). 2.5. Synthesis of Silica Encapsulated QDs−EVA Nanocomposites. The synthesis of nanocomposites was carried out by using both one-pot and two-step processes. In the one-pot process, functionalization of NPs was carried out during polymerization, while in the two-step process, prefunctionalized NPs (as described in section 2.4) were added with monomers before polymerization was initiated. The polymerization experiments were conducted in a 100 mL high-pressure stainless steel autoclave (Parr Instrument Company, USA). The reactor was coupled with a temperature controller and a digital pressure transducer. The stirring speed was maintained at a constant speed of 300 rpm. In a typical one-pot method, 9.22 mL of vinyl acetate (monomer), 0.9 mL of DEPDC (initiator), 50 mg of silica encapsulated CdS−ZnS QDs, 0.77 mL of VMTS (linker), and 1.14 mL of glacial acetic acid (as hydrolysis agent) were added into the autoclave, followed by an argon purging. Then the second monomer, ethylene (1.07 mol), was pumped in using a syringe pump (Isco 100 DX). After that, CO2 was pumped into the reactor using 3188

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Figure 1. FTIR spectra of (a) silica encapsulated CdS/ZnS QDs (i) as synthesized and (ii) vinyl functionalized; (b) polymer nanocomposites synthesized under scCO2 (iii) EVA copolymer and (iv) EVA nanocomposites incorporated with vinyl functionalized silica encapsulated CdS/ZnS QDs using the two-step process.

Figure 2. (a) TG and (b) DTG curves of (i) silica encapsulated CdS/ZnS QDs and (ii) silica encapsulated CdS/ZnS QDs after vinyl functionalization; (iii) EVA copolymer and (iv) EVA nanocomposites grafted with silica encapsulated CdS/ZnS QDs using two-step process. absorption spectra (wavenumber range: 500−4000 cm−1) using a Fourier-transform infrared spectroscope (Nicolet 6700 FTIR). Thermogravimetric (TG) and derivative thermogravimetric (DTG) analysis was carried out on a SDT Q600 instrument. The analysis was performed under a nitrogen atmosphere at a heating rate of 10 °C/ min for a temperature range of 30−700 °C. A UV−vis−NIR spectrophotometer (Shimadzu UV-3600) was used to observe both the UV and visible light transmission through the EVA nanocomposites films of variable thickness. An integrating sphere compartment was used to measure total transmission and diffused transmission. Then haze was calculated from the ratio of diffused to the total transmission. Thermicity, the IR transmission through the EVA nanocomposite films, was determined from the FTIR absorbance (700−1400 cm−1) as explained previously.36 Heat conduct ability and thermal conductivity of the neat EVA and EVA nanocomposites were determined using a previously developed experimental setup.37 The tensile test was carried out on an Instron universal test instrument (Model 5943, Norwood, MA) at room temperature. Rectangular samples of dimension (L × W × T = 70 mm × 10 mm × 2 mm) were analyzed at loading rate of 50 mm/min. The average value and standard deviations were calculated based on testing more than five samples.

consistent with the literature.38 These significant peaks remained after vinyl functionalization shown in Figure 1a(ii). After vinyl functionalization, new peaks were observed at 2950 and 2860 cm−1, assigned to the alkene C−H vibration and the aliphatic C−H vibrations, respectively.35 A small peak at 1604 cm−1 is evident that represents CC stretching vibration in the vinyl functionalized samples. As seen in Figure 1b(iii), the major peaks for EVA copolymer were observed at 2930, 2845, and 1470 cm−1 for aliphatic C−H vibrations, 1737 cm−1 for CO stretching vibration, and 1235 and 1022 cm−1 due to C−O−C stretching vibrations. These peaks match with previous reports for EVA.8,39 All these significant peaks were also observed after nanocomposite formation, indicating successful integration of the QDs into the EVA polymer matrix. A new band appearing at 1090 cm−1 and a small peak at 782 cm−1 (indicated with blue star in Figure 1b(iv)) are attributed Si−O−Si vibrations, justifying the silica presence in the EVA nanocomposites. The lower intensity of these peaks is due to low loading concentration of QDs during synthesis of the nanocomposites. In addition, any weak peak at 1604 cm−1 from the functionalized QDs is not here, which confirms complete reaction. Thermogravimetry (TG) and differential thermogravimetry (DTG) analyses for silica encapsulated CdS/ZnS QDs before and after vinyl functionalization are shown in Figure 2a,b (lines i,ii). The weight loss of NPs was about 17.8% for the experimental temperature range (30−700 °C), which is largely

3. RESULTS AND DISCUSSION 3.1. Structural Characteristics. Silica encapsulated CdS/ ZnS QDs before and after surface functionalization were characterized by FTIR. As shown in Figure 1a(i), a strong peak at 1100 cm−1 and weaker peak at 790 cm−1 were seen for silica encapsulated QDs due to Si−O−Si vibrations, which is 3189

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Figure 3. SEM images of (a) silica encapsulated CdS/ZnS QDs after vinyl functionalization, (b) EVA copolymer, (c, d) EVA nanocomposites grafted with vinyl functionalized silica encapsulated CdS/ZnS QDs using two-step polymerization process, (e) EVA nanocomposites using the onepot process, (f) SEM-EDX of EVA nanocomposites using the two-step process, and (g) SEM-EDX of EVA nanocomposites using the one-pot process.

Figure 4. TEM images of (a) CdS/ZnS QDs, (b) silica encapsulated CdS/ZnS QDs after vinyl functionalization, (c) neat EVA, (d) EVA nanocomposites grafted with silica encapsulated CdS/ZnS QDs using one-pot process, and (e) EVA nanocomposites using the two-step process.

of surface functionalization agents.19 TG and DTG thermographs of EVA copolymer synthesized under scCO2 are also shown in Figure 2a,b. There were two stages of weight loss observed at approximately 360 and 475 °C. The first stage is attributed to the copolymer losing acetic acid, and from this mass loss the wt % of VA (vinyl acetate) can be calculated.41 The weight loss for the first stage was approximately 23.4%. By

due to dehydration of free water (adsorbed from atmosphere) and any structural water, if present, and the dehydroxylation of silanol groups from the silica shell.40 An additional 7.8% weight loss (showing a sharp change at 560 °C) representing the decomposition of the attached organic units on the silica surface after vinyl functionalization. Similar weight loss has been reported for silica NPs functionalization using other types 3190

DOI: 10.1021/acsanm.8b00390 ACS Appl. Nano Mater. 2018, 1, 3186−3195

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narrow size distribution. The average diameter of the nanospheres was 60 ± 5 nm shown in the histogram data with a core size (mostly QDs) 35−40 nm. This size range is consistent with the particle size of the silica encapsulated CdS/ ZnS QDs reported previously.9 Figures 4c−e show the TEM images of neat EVA copolymer and EVA nanocomposites with incorporation of functionalized silica encapsulated QD NPs. It is clearly observed that the QDs are rather well dispersed in the EVA matrix. However, in the case of nanocomposites prepared following the two-step process a better homogeneous dispersion was observed without showing any signs of aggregation (Figure 4e). The two-step approach can efficiently utilize the surface functionalization ligand. In this case, the functionalized QD NPs were well dispersed in solvent before adding with monomer/initiator mixture, providing an enhanced compatibility and chemical linking between the polymer network and the NPs. The NPs distribution observed from the TEM analysis also corroborated the findings from SEM analysis. As electron microscopy imaging methods (SEM, TEM) only provide an indication of NP distribution on the surface plane only, the QDs distribution throughout the EVA polymeric matrix was examined using a laser scanning confocal microscopy (LSCM). By preparing a 250 μm thickness film, the focal planes of the EVA nanocomposite films were excited using an argon laser, with the resulting QDs emission showing as bright green dots. Figure 5 shows confocal images of a

multiplying this value by the ratio of the molecular weight of VA monomer to the molecular weight of acetic acid, the wt % of VA of the synthesized EVA copolymer was calculated as 33.5%. In the second stage, the hydrocarbon chain decomposes to simple hydrocarbon (CH2CH2, CH4) and eventually to CO2. Therefore, the weight was around zero. In the case of EVA nanocomposites with functionalized QDs as shown in Figure 2a(iv), the thermograph was mostly similar as for the net EVA (Figure 2a(iii)) because of the very low loading of QDs (∼1%). After 500 °C, for EVA nanocomposites prepared by both a one-pot and two-step approach, the remaining final weight is around 1%, which is the final loading of the QD after complete decomposition of the polymer matrix. 3.2. Morphology of Silica Encapsulated QDs and Their Dispersion in EVA. To improve compatibility of silica encapsulated CdS/ZnS QDs and their binding with EVA, encapsulated QDs were functionalized with vinyltrimethoxysilane (VTMS) as shown by Scheme 1, step 2. The one-pot process provides functionalization of silica encapsulated QDs and grafting-f rom polymerization of EVA at the same time in the same reactor. The challenge of this process lies in the limited solubility of the QDs in the vinyl acetate/initiator mixture. To minimize this, in the two-step process, silica encapsulated QDs samples were dispersed in glacial acetic acid that was used as a hydrolyzing agent during graf ting-from polymerization and vinyl functionalization was carried out before beginning the polymerization. Scanning electron microscopy (SEM) imaging shows the morphologies of the functionalized NPs, EVA copolymer, and EVA nanocomposites. Figure 3a presents the silica encapsulated QDs after vinyl functionalization. These modified QDs were formed as smooth nanospheres with less than 100 nm size. The surface features of EVA copolymer (Figure 3b) and EVA nanocomposites incorporated with these functionalized silica encapsulated QDs using the two-step polymerization approach (Figure 3c) were the same. No significant NP agglomeration was observed on the polymer surface and also inside of the nanocomposites samples seen through cracks and edges. During SEM-EDX analysis, except for carbon there were no significant peaks seen due to the very low concentration (∼1%) of QDs and their good dispersion (Figure 3f). The morphology of the EVA nanocomposites using the onepot process (Figure 3e) was also consistent with that for EVA virgin polymer (Figure 3b). However, as shown in Figure 3e, there were a few bright spots observed on the polymer surface which were further analyzed by EDX. EDX showed the presence of Si and O from the QD silica shell, while Zn, S, and Cd from the QDs were also found (Figure 3g). These elements suggest that the QD content varied from one region to the next in the EVA matrix because of the size and density of the agglomeration. Transmission electron microscopy (TEM) was used to better examine the size and size distribution of QDs and functionalized silica NPs as well as to investigate the distribution of these NPs throughout the EVA copolymer matrix. Figure 4a shows a TEM image of the CdS/ZnS QDs, showing formation of individual nanocrystals. The average particle size was calculated as 5 ± 0.5 nm by ImageJ software and shown in the histogram data. Here, the individual NPs formation was because of the thin ZnS shell that minimizes the aggregation tendency and surface crystal defects of bare CdS.8 As shown in Figure 4b, the silica encapsulated CdS/ZnS QDs after vinyl functionalization forms uniform nanospheres with

Figure 5. Laser scanning confocal microscopy imaging of EVA nanocomposites films loaded with vinyl functionalized silica encapsulated CdS/ZnS QDs prepared using (a) one-pot process and (b) two-step process.

random plane approximately at the middle thickness of the nanocomposites films. These images show that the bright green dots were well dispersed, confirming the uniform distribution of the silica encapsulated CdS/ZnS QDs throughout the EVA nanocomposites prepared using both the one-pot and two-step approaches. However, consistent with the TEM results, the EVA nanocomposites prepared using the two-step process showed a better homogeneous distribution of the silica encapsulated QDs throughout the EVA network (Figure 5b). Polymer/inorganic nanocomposites synthesized using the conventional solution mixing process suffer aggregation of filler particles because of slow solvent evaporation, and to get even a milligram of products can take more than a day.42,43 In the two-step approach, not only toxic organic solvents were avoided, in the presence of scCO2, the filler NPs were homogeneously dispersed throughout the polymer matrix. 3191

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ACS Applied Nano Materials Table 1. Optical Properties of EVA and EVA Nanocomposite Synthesized under scCO2 sample

thickness of films (μm)

% T (UV) (at 260 nm)

% T (visible)(at 550 nm)

% haze (at 550 nm)

EVA

100 250 500

98.1 ± 0.3 96.8 ± 0.4 93.3 ± 0.8

98.0 ± 0.2 95.8 ± 0.3 92.4 ± 0.6

17.5 ± 0.5 18.9 ± 0.5 21.3 ± 0.6

EVA nanocomposites (one-pot process)

100 250 500

83.3 ± 0.5 70.2 ± 0.4 36.5 ± 0.8

96.3 ± 0.5 92.6 ± 0.8 88.3 ± 1.2

33.8 ± 0.4 36.2 ± 0.6 41.3 ± 1.0

EVA nanocomposites (two-step process)

100 250 500

72.4 ± 0.6 50.4 ± 0.5 20.5 ± 1.0

96.6 ± 0.5 94.3 ± 0.6 90.2 ± 0.8

31.3 ± 0.5 33.5 ± 0.5 39.1 ± 0.8

Figure 6. (a) Total light transmission in UV and visible region of (i) EVA copolymer and EVA nanocomposites loaded with vinyl functionalized silica encapsulated CdS/ZnS QDs prepared using the (ii) one-pot process and (iii) two-step process (film thickness 250 μm). (b) Thermicity and thermal conductivity of EVA copolymer and EVA nanocomposite films. Film thickness for thermicity study was 100, 250, and 500 μm and for thermal conductivity was 500 μm.

problems, the surface passivation and functionalization of QDs is very crucial which minimizes the interface energy between the QD and the polymer matrix and also aggregation tendency among QDs.19,20 As shown in Figures 4 and 5, the two-step approach of graf ting-f rom polymerization provided enhanced dispersion in the monomer/initiator mixture. This approach results in the most uniform dispersion of silica encapsulated CdS/ZnS QDs in the EVA network compared to the nanocomposite prepared using the one-step polymerization or prepared using a melt-mixing approach.9 3.3. Optical and Thermal Properties of EVA Nanocomposites. The optical properties of the EVA copolymer and EVA nanocomposites synthesized using both the one-pot and two-step processes were examined using an UV−vis−NIR spectrophotometer coupled with an integrated sphere compartment. The total light transmission in the UV and visible region and the diffused transmission were investigated for thin EVA nanocomposite films of variable thickness (100− 500 μm). The results are summarized in Table 1. Neat EVA copolymer films showed high transparency of visible light. The transmittance decreased from 98% to 92% with increasing film thickness, which is justified with the Beer−Lambert law. Although the films showed high transparency, the UV transmission was also very high (more than 93%). Highenergy UV radiation causes rapid degradation of the polymer network that limits short and long span practical applications of polymer composites.6 The EVA nanocomposites incorporated with functionalized silica encapsulated CdS/ZnS QDs prepared using both the one-pot and two-step processes under scCO2 show a very little decrease in the visible light

Also, this approach is more viable and economically feasible for large-scale industrial production. The synthesis technique of the polymer nanocomposite plays a critical role for uniform dispersion of NPs in the polymeric network. Polymerization in the presence of inorganic NPs approach shows better dispersion than the blending method, although blending is a simple and convenient approach for preparing nanocomposites. The high surface energy of the QD materials and their low compatibility with polymer restrict them to maintain their good dispersibility within the polymer matrix.1,16 The compatibility of QDs with polymer can be improved by surface functionalization and encapsulation of QDs and also homogeneous mixing of QDs in a solution of monomers/ oligomers/initiator.6,9,16 During polymer nanocomposites synthesis using the in situ method, the inorganic NPs are nucleated and grown inside the polymer network which forms from the monomers simultaneously. The benefits of this approach include avoiding the isolation and handling of NPs and not needing separate passivation or stabilization steps by functionalization of NPs.17,19 However, the major limitation of this approach is to control the functionalization during polymerization and the remainder of unreacted reactants/precursors or byproducts with the products that might influence the properties of the nanocomposite products.16 Compared to the in situ technique, graf ting polymerization on preprepared QDs is a very promising and versatile method. The key demand of this approach is to control the aggregation tendency and stability of QDs during the polymerization process. To solve these 3192

DOI: 10.1021/acsanm.8b00390 ACS Appl. Nano Mater. 2018, 1, 3186−3195

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Figure 7. (a) Typical stress−strain curves of (i) EVA copolymer and EVA nanocomposites loaded with vinyl functionalized silica encapsulated CdS/ZnS QDs prepared using the (ii) one-pot process and (iii) two-step process. (b) Tensile strength, Young’s modulus, and elongation at break of EVA copolymer and EVA nanocomposites loaded with vinyl functionalized silica encapsulated CdS/ZnS QDs.

losses. As shown in Figure 6b, the thermicity of the EVA copolymer film is very high and dependent on the thickness of the films. Higher thermicity is undesired for solar coverings as it gives a greater rate of radiative heat loss by infrared transmission through the films. For the case of EVA nanocomposite films prepared by both the one-pot and twostep processes, the thermicity values were significantly reduced. The reason behind this is the presence of silica NPs. Nanosilica has the potentiality to block infrared transmittance without losing visible light transmission.8 EVA nanocomposites gave similar thermal conductivity trend as thermicity as presented in Figure 6b. As previously reported, these silica NPs contains mesoporous structure with pore size 3.8 ± 0.4 nm by BET analysis.9 So, silica increases the porosity and voids in the nanocomposite network. Silica nanopores also decrease the radiative heat transfer and gas conduction by scattering or reflecting the thermal waveband paths.8 Uniform dispersion of silica NPs further improves both the thermicity and thermal conductivity performances in EVA nanocomposites prepared by the two-step process. Therefore, mesoporous silica encapsulation on QDs not only improves the compatibility of inorganic NPs with polymer and optical properties of nanocomposites but also reduces the radiative heat transmission through the nanocomposite films. 3.4. Mechanical Properties. Figure 7a shows typical stress−strain curves of EVA copolymer and EVA nanocomposites prepared by the one-pot and two-step processes. Compared to pure EVA, the EVA nanocomposites incorporated with vinyl functionalized silica encapsulated QDs show an increased tensile stress and decreased elongation at break. These trends are consistent with the mechanical properties reported previously for EVA composites with other inorganic NPs or silica reinforcement.48,49 The average Young’s modulus, tensile strength, and elongation at break were calculated and are presented in Figure 7b. Compared to EVA copolymer, there was a tensile strength increment of 16% and 38% for EVA nanocomposites by the one-pot and two-step processes, respectively. The Young modulus was also increased 36.4% and 43.2% after NP loading in EVA. Elongation at break decreased after nanocomposite formation; however, the improved QD dispersion enhanced the elongation at break, which is consistent with the literature.48 Compared to pure EVA, both types of EVA nanocomposites show improved mechanical properties. Between both EVA nanocomposites, EVA nanocomposites prepared by the two-step approach showed better

transmission (88−97%). However, in comparison to the previously reported commercial greenhouse EVA plastic films, the visible transmittances of the experimental QDs− EVA nanocomposite films are higher.6 While the visible light transparency of the experimental films remained very high, almost the same as plain EVA, a prominent decreasing trend was observed in the UV region. For instance, a 250 μm thick EVA nanocomposite film thickness prepared using the one-pot approach shows UV transmission values around 70% compared to 97% for neat EVA film (Figure 6a). This UV shielding performance was further improved (around 50%) in the case of EVA nanocomposite prepared using the two-step process. The lowering of this UV transmission is because of the absorption of higher energy UV radiation by CdS/ZnS QDs that can be efficiently converted into desirable lower energy visible light through emission, and silica shell is transparent on UV−vis transmission of EVA nanocomposites.8 It is attributed that the uniform distribution of the QDs throughout the polymer network as shown in Figures 4 and 5 enhances UV blocking by absorption. The UV shielding performance can be further tuned by changing film thickness. Because of silica encapsulated QDs dispersion in the EVA network, the diffuse transmission (haze) was also increased from 20 to 40% (Table 1). Compared to direct light, a controlled increase of the diffuse light is advantageous for both the greenhouse production and PV module performance.44,45 The visible transmittance, UV shielding, and haze of the experimental EVA nanocomposite films were superior to similar films prepared by melt-mixing using extrusion or loaded with bare and functionalized QDs.6,9 Here, silica encapsulation not only enhances quantum yield of QDs but also provides a platform for functionalization to improve the linking and distribution to EVA copolymer. In this work, we have also examined the thermal performance measuring the thermicity (IR transmission) and thermal conductivity of the EVA nanocomposite films. Controlling the radiative heat loss through long-wave infrared and thermal wavebands is an ongoing drawback of the solar harvesting films. This also adversely affects material life span, biomass production in greenhouses, or PV efficiencies by downregulation.46,47 Commercial thermic films can minimize heat losses to some extent, but due to large particle size of the additives visible light transmission reduces significantly.8 The thermal conductivity and thermicity of the nanocomposite films play a vital role to assess the heat retention and heat 3193

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(5) Agroui, K.; Collins, G. Characterisation of EVA encapsulant material by thermally stimulated current technique. Sol. Energy Mater. Sol. Cells 2003, 80, 33−45. (6) Allan, J. M.; Abdul Mumin, M.; Xu, W. Z.; Al Sharari, Q.; Charpentier, P. A. Surface functionalized bare and core−shell quantum dots in poly (ethylene-co-vinyl acetate) for light selective nanocomposite films. Sol. Energy Mater. Sol. Cells 2014, 123, 30−40. (7) Gao, Y.; Yao, W.; Sun, J.; Zhang, H.; Wang, Z.; Wang, L.; Yang, D.; Zhang, L.; Yang, H. A novel soft matter composite material for energy-saving smart windows: from preparation to device application. J. Mater. Chem. A 2015, 3, 10738−10746. (8) Mumin, M. A.; Akhter, K. F.; Dresser, S.; van Dinther, S. T.; Wu, W.; Charpentier, P. A. Multifunctional mesoporous silica nanoparticles in poly (ethylene-co-vinyl acetate) for transparent heat retention films. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 851−859. (9) Mumin, M. A.; Xu, W. Z.; Charpentier, P. A. Quantum dots/ silica/polymer nanocomposite films with high visible light transmission and UV shielding properties. Nanotechnology 2015, 26, 315702−315715. (10) Martínez-Ferrero, E.; Albero, J.; Palomares, E. Materials, nanomorphology, and interfacial charge transfer reactions in quantum dot/polymer solar cell devices. J. Phys. Chem. Lett. 2010, 1, 3039− 3045. (11) Xu, W. Z.; Charpentier, P. A. Light-selective nanofilms of quantum dot-poly (ethylene-co-vinyl acetate) synthesized with supercritical CO2. J. Phys. Chem. C 2009, 113, 6859−6870. (12) Coropceanu, I.; Bawendi, M. G. Core/shell quantum dot based luminescent solar concentrators with reduced reabsorption and enhanced efficiency. Nano Lett. 2014, 14, 4097−4101. (13) Han, H.; Francesco, G. D.; Maye, M. M. Size control and photophysical properties of quantum dots prepared via a novel tunable hydrothermal route. J. Phys. Chem. C 2010, 114, 19270− 19277. (14) Alivisatos, A. P.; Gu, W.; Larabell, C. Quantum dots as cellular probes. Annu. Rev. Biomed. Eng. 2005, 7, 55−76. (15) Bera, D.; Qian, L.; Tseng, T.-K.; Holloway, P. H. Quantum dots and their multimodal applications: a review. Materials 2010, 3, 2260−2345. (16) Lü, C.; Yang, B. High refractive index organic−inorganic nanocomposites: design, synthesis and application. J. Mater. Chem. 2009, 19, 2884−2901. (17) Porel, S.; Singh, S.; Harsha, S. S.; Rao, D. N.; Radhakrishnan, T. Nanoparticle-embedded polymer: in situ synthesis, free-standing films with highly monodisperse silver nanoparticles and optical limiting. Chem. Mater. 2005, 17, 9−12. (18) Sadeghi, M.; Khanbabaei, G.; Dehaghani, A. H. S.; Sadeghi, M.; Aravand, M. A.; Akbarzade, M.; Khatti, S. Gas permeation properties of ethylene vinyl acetate−silica nanocomposite membranes. J. Membr. Sci. 2008, 322, 423−428. (19) Kim, Y.-J.; Kim, J.-H.; Ha, S.-W.; Kwon, D.; Lee, J.-K. Polyimide nanocomposites with functionalized SiO2 nanoparticles: enhanced processability, thermal and mechanical properties. RSC Adv. 2014, 4, 43371−43377. (20) Ji, X.; Hampsey, J. E.; Hu, Q.; He, J.; Yang, Z.; Lu, Y. Mesoporous silica-reinforced polymer nanocomposites. Chem. Mater. 2003, 15, 3656−3662. (21) Wei, L.; Hu, N.; Zhang, Y. Synthesis of polymermesoporous silica nanocomposites. Materials 2010, 3, 4066−4079. (22) Corbierre, M. K.; Cameron, N. S.; Sutton, M.; Laaziri, K.; Lennox, R. B. Gold nanoparticle/polymer nanocomposites: dispersion of nanoparticles as a function of capping agent molecular weight and grafting density. Langmuir 2005, 21, 6063−6072. (23) Tang, E.; Cheng, G.; Ma, X. Preparation of nano-ZnO/PMMA composite particles via grafting of the copolymer onto the surface of zinc oxide nanoparticles. Powder Technol. 2006, 161, 209−214. (24) Zhao, Q.; Samulski, E. T. In situ polymerization of poly (methyl methacrylate)/clay nanocomposites in supercritical carbon dioxide. Macromolecules 2005, 38, 7967−7971.

mechanical performance which is attributed to the homogeneous dispersion of functionalized silica encapsulated CdS/ ZnS QDs and the higher compatibility between QDs and EVA molecular chain.48,49 Enhanced dispersion and compatibility also facilitate effective stress transfer in the polymer nanocomposites.48

4. CONCLUSIONS A graf ting-from polymerization of EVA under supercritical CO2 has been developed where polymer chains from the monomers ethylene and vinyl acetate grow on the functionalized silica coated CdS/ZnS QDs. The core/shell CdS/ZnS QDs with an average size of 5 nm were prepared using a colloidal synthesis from single-molecule precursors. Thin layer of ZnS minimized the aggregation tendency and photooxidation of core CdS. Then encapsulation of the QDs by a control layer of mesoporous silica by a reverse microemulsion technique enhanced the quantum efficiency and photostability of CdS/ZnS QDs. Silica surface of the QDs was then functionalized to introduce vinyl groups by vinyltrimethoxysilane, a silane linking agent. The functionalization and polymerization were explored both by the one-pot (together) and two-step (separately) processes. The silica encapsulated QDs synthesized by the two-step approach showed excellent compatibility and dispersion throughout the EVA network by TEM, SEM, and CLSM analysis. The novel EVA nanocomposites also showed improved optical and thermal properties with high UV shielding, visible light transmission, and thermal insulation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel (519) 661-3466; Fax (519) 661-3498 (P.A.C.). ORCID

Md Abdul Mumin: 0000-0003-1717-1538 William Z. Xu: 0000-0002-3460-8474 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), Canadian Natural Science and Engineering Research Council (NSERC) Discovery grant, ReMAP Network, and MITACS-Accelerate Internship, Canada. Authors also thank Dr. Lijuan Wang for her contribution during setup of polymerization reactor system using supercritical CO2.

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