Department of Intelligent Energy and Industry, School of Chemical Engineering and Materials Science, Institute of Energy-Converting Soft Materials, Chung-Ang University, Seoul 06974, Korea
*KNW R&D Center, Donyu 2-ro 60, Paju-eup, Paju-city, Gyeonggi-do 10832, Korea
중앙대학교 지능형에너지산업학과, 화학신소재공학부, *KNW연구소
Reproduction, stored in a retrieval
system, or transmitted in any form of any part of this
publication is permitted only by written permission from the
Polymer Society of Korea.
Hybrid nanoparticles based on tungsten-doped vanadium dioxide (WxVO2) nanoparticles and poly(N-isopropylacrylamide) (PNIPAm) for thermochromic smart window applications were synthesized via silica layer formation using (3-mercaptopropyl)trimethoxysilane (MPTMS) followed by the surface-mediated free radical polymerization of PNIPAm, resulting in the aqueous dispersion of the hybrid nanoparticles. The metal-insulator transition of the WxVO2 core at around 28 °C and the lower critical solution temperature of PNIPAm at around 32 °C enable the aqueous dispersion of the hybrid nanoparticles to present an efficient thermochromicity depending on the temperature over a broad spectral range from visible to near-infrared light. The difference in the solar modulation was over 25%, while the luminous transmittance (Tlum) significantly decreased from 85% to 15% with increasing WxVO2 composition in the hybrids. Our results suggest that the hybridization of WxVO2 and PNIPAm is a potential methodology for preparing efficient thermochromic materials
열변색 스마트 창호 소재로서 텅스텐이 도핑된 바나듐이산화물(WxVO2) 나노입자와 폴리이소프로필아크릴아미드(PNIPAm)의 하이브리드 나노입자가 실리카 쉘 형성 및 PNIPAm의 표면라디칼 중합반응에 의해 합성되었다. 하이브리드 나노입자의 수분산액은 섭씨 28도의 금속-절연체 전이온도를 보유한 WxVO2 코어와 섭씨 32도의 하한 임계 용액온도를 보유한 PNIPAm 쉘의 특성으로 인해 가시광선에서 근적외선까지의 넓은 스펙트럼 범위에서 효과적인 열변색 특성을 보였다. 하이브리드 나노입자에서 PNIPAm의 함량을 고정시키고 WxVO2의 함량을 변화시키며 특성을 측정한 결과 섭씨 20도에서 40도의 온도 변화에서 25% 이상의 태양광 변조 변화를 구현하였으며 WxVO2의 함량증가에 따라 입자의 증가된 뭉침현상에 의해 투과도는 85%에서 15%까지 급격하게 감소하였다. 본 연구결과는 WxVO2와 PNIPAm의 하이브리드화가 효율적인 열변색 소재를 제조하기 위한 주요 기술이 될 수 있음을 보여준다
Keywords: vanadium oxide, tungsten doping, poly(N-isopropylacrylamide), thermochromic, smart windows
This research was supported by the Chung-Ang University Graduate Research Scholarship in 2019 and the Korea Institute of Energy Technology Evaluation and Planning, funded by the Ministry of Trade, Industry and Energy (Grant No. 20182020109500), Republic of Korea.
Nowadays, technologies for saving energy are receiving widespread attention, with many materials and methods having been studied. Among them, thermochromic smart windows that can control the transmission of sunlight are popular energy-saving devices in buildings because energy consumption used for buildings accounts for 30-40% of the total energy consumption.1,2 Energy can be saved by efficiently manipulating the amount of sunlight transmitted through the smart window.3 For example, when the outside temperature is high, the smart window can reduce the amount of sunlight entering the building, thereby decreasing the cost of cooling the interior of the building. On the other hand, when the outside temperature is low, the smart window can transmit the sunlight, thus maximizing energy-saving efficiency through internal temperature control.4
A representative thermochromic material used for smart windows is monoclinic vanadium dioxide (VO2(M)). It presents a metal-insulator transition (MIT) at around 68 ℃, above which its structure changes from monoclinic to tetragonal and is accompanied by a change in the electrical property of VO2 from semiconducting to metallic.5,6 The resulting reflection of near-infrared light (NIR) at a temperature above 68 ℃ enables smart windows utilizing VO2 to present the highest solar modulation ability (Tsol) of 22.3% and an average luminous transmission (Tlum, average) of 42.8%.7,8 However, the MIT temperature of pristine VO2(M) is too high for practical smart window applications that require an MIT at around room temperature and also the thermochromic property is limited to the NIR range.9,10 Thus, a new technology based on VO2 with an MIT near room temperature and solar modulation capability in the visible-to-NIR range is strongly required.
A promising approach to resolve this issue is to combine tungsten-doped VO2 (WxVO2) with a thermo-responsive polymer that can modulate sunlight transmission in the visible light range. When W6+ or Ti4+ is incorporated into the crystalline structure of VO2, the donation of electrons and distortion of the crystalline structure results in a decrease in the MIT temperature of pristine VO2(M) down to room temperature.11-13 On the other hand, poly(N-isopropylacrylamide) (PNIPAm), an organic thermo-responsive polymer, has a lower critical solution temperature (LCST) at around 32 ℃ and has received wide attention in smart window applications due to their change of transparency from a transparent state to an opaque state when the temperature changes above the LCST.14-19 Because of the aggregation and precipitation of polymer chains above the LCST, the aqueous solution of PNIPAm becomes cloudy, thereby blocking the transmission of visible light, which is useful for smart window applications. For example, a PNIPAm hydrogel film shows a high Tlum of 70.7% and DTsol of 25.5% in the visible range.17 Thus, one can expect that the hybridization of WxVO2 with PNIPAm will provide solar modulation capability in the visible-to-NIR range at around 30 ℃, which should be useful for practical applications of smart windows. The hybridization of VO2 and PNIPAm has already been reported.7,14,17 However, those studies focused on microgel and film forms that are different from the aqueous solution systems in our study, and the hybridization of WxVO2 with PNIPAm has not yet been reported.
In this study, we demonstrate the fabrication and thermochromicity of hybrid nanoparticles comprising WxVO2 and PNIPAm prepared via the surface polymerization of PNIPAm on WxVO2 nanoparticles in Scheme 1. Firstly, the WxVO2 nanoparticles was synthesized by hydrothermal synthesis with sodium tungstate. And then, a silica layer with surface thiol groups was first formed on the WxVO2 nanoparticles via the functionalization with (3-mercaptopropyl)trimethoxysilane (MPTMS) to provide surface thiol functionalities for polymerization and chemical stability20 to the core WxVO2 under exposure to aqueous media. PNIPAm was then synthesized on the silica shell surface via free radical polymerization. The resulting hybrid organic-inorganic nanoparticles (WxVO2@ MPTMS@PNIPAm) dispersed in an aqueous medium presented a reversible phase-transition property from a transparent (hydrophilic) to an opaque (hydrophobic) state at its LCST temperature, which is characteristic of PNIPAm.14 Moreover, they presented an efficient solar modulation ability over the NIR range at around room temperature, which is characteristic of WxVO2. The aqueous dispersion of WxVO2@MPTMS@ PNIPAm hybrid nanoparticles suggests the facile fabrication of smart windows by injecting the solution into an empty narrow space between two glass windows, which should be beneficial for simple, cost-effective fabrication.
Scheme 1. Synthesis and coating procedure for WxVO2, WxVO2@ MPTMS, and WxVO2@MPTMS@PNIPAm.
Materials.Oxalic acid dihydrate (C2H2O4∙2H2O), vanadium pentoxide (V2O5), sodium tungsten oxide dihydrate (Na2WO4∙ 2H2O) were purchased from Alfa Aesar (Heysham, UK). N-isopropylacrylamide (NIPAm) and 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrate (VA044, C12H22N6∙2HCl) were purchased from the Tokyo Chemical Industry (TCI, Tokyo, Japan). Polyvinylpyrrolidone (PVP, MW~1300000 Da) and MPTMS (SH(CH2)3Si(OCH3)3) were purchased from Sigma-Aldrich (St. Louis, MO, USA). All of the reagents were used as received.
WxVO2 Nanoparticles Synthesis.Oxalic acid (10.08 g, 0.112 mol) and V2O5 (7.27 g, 0.04 mol) were dissolved in deionized (DI) water (100 mL) in a three-neck round-bottomed flask equipped with a condenser for reflux. Na2WO4 (0.29 g; 2 mol% of W to V) was dissolved in 10 mL of DI water. The mixture was stirred at 400 rpm for 24 h at 60 °C. Afterward, the mixture was transferred to a Teflon-lined autoclave and the hydrothermal reaction was conducted at 185 °C for 12 h. After completing the reaction, the product was washed by centrifuging with ethanol three times and dried in a vacuum oven at room temperature for 24 h. The dried black-blue powder was calcinated at 700 °C for 2 h under a nitrogen atmosphere of a furnace.
WxVO2@MPTMS Coating. Wx-VO2 (0.05 g) dispersed in ethanol (5 mL) via sonication for 30 min and PVP (0.128 g, 1.2 mmol on a repeat unit basis) dissolved in ethanol (10 mL) were added to a round-bottomed flask and stirred at room temperature for 72 h. Next, MPTMS solution (0.5 mL dissolved in 10 mL of ethanol) and ammonium solution (10 mL, 35 wt%) were added and stirred at room temperature for 12 h, resulting in a gray mixture solution. The precipitate was collected by centrifuging the solution at 8000 rpm for 15 min with excess ethanol to remove any uncoated WxVO2. The powder was dried in a vacuum at room temperature for 24 h.
WxVO2@MPTMS@PNIPAm Synthesis.In a three-neck round flask, WxVO2@MPTMS powder (0.05 g) was dispersed in DI water (10 mL) via sonication for 30 min, followed by the addition of VA044 (0.05 g, 0.155 mmol) in DI water (10 mL) and NIPAm (1.13 g; 10 mmol) in DI water (10 mL). The resulting solution was stirred at 60 °C for 4 h with nitrogen bubbling. Afterward, the solution mixture was precipitated in excess methanol (100 mL) to obtain a sticky solid product comprising black WxVO2 coated with white PNIPAm synthesized on the nanoparticle surfaces. This product was dried in a vacuum at room temperature for 24 h, which resulted in a hard product.
Characterization.Morphological observations of the nanoparticles shell were conducted by using a field-emission transmission electron microscope (FE-TEM; JEM-F200, JEOL, Japan). The crystalline structures of WxVO2(M) and VO2(M) were verified via X-ray diffraction (XRD; Bruker-AXS NEW D8 Advance, Billerica, MA, USA). The MIT behaviors of WXVO2(M) and VO2(M) were investigated using differential scanning calorimetry (DSC; Q-600, TA Instruments, Newcastle, DE, USA) analysis in the range of 20-130 °C at a heating rate of 10 °C/min. The thermochromic properties of WxVO2(M) and WxVO2@MPTMS@PNIPAm were measured with a UV-Vis-NIR spectrometer (V-770, JASCO, Tokyo, Japan) in the range of 300-2500 nm. Thermal decomposition behaviors were analyzed by thermogravimetric analysis (TGA; TA SDT Q600, TA Instruments, Newcastle, DE, USA) in the range of 20-600 °C at a heating rate of 10 °C/min.
The structures and phase-transition behavior of VO2 and WxVO2 were analyzed via measurements from XRD patterns and DSC thermograms. The diffraction peaks in the observed XRD patterns of both VO2 and WxVO2 were accurately matched with the monoclinic VO2 crystalline structure (JCPDS NO.43-1051).21 In the XRD patterns, sharp diffraction peaks for VO2(M) at 26.8°, 27.8°, 33.4°, 37.0°, 39.8°, 42.1°, 42.2°, 55.5°, and 57.4°, corresponding to the (110), (011), (-102), (-202; -211; 200), (002), (-212;210), (-213; 220; 211), and (022) planes, show the high crystallinity of VO2 and WxVO2 prepared in this study (Figure 1). The phase-transition behavior observed on the DSC thermograms clearly indicates the tungsten doping effect. When 2 mol% of tungsten to vanadium was added, the phase-transition temperature decreased from 66.9 to 27.9 ℃, as shown in Figure 2(b). These results clearly show that although the small amount of tungsten incorporated into the pristine crystalline structure of VO2 did not significantly modify the original structure, it did nevertheless decrease the MIT temperature to around room temperature. The shoulder peak appeared at around 80 °C in the DSC thermogram of VO2 is ascribed to phase transition of agglomerate of particles. It should be noted that the shoulder peak is significantly weakened after multiple heating and cooling cycles as shown in Figure 2(b) and that the phase transition temperatures are slightly shifted to 29.7 and 67.9 °C for WxVO2 and VO2, respectively. These results suggest that the agglomerates might be loosened and that the particles have stable phase transition behaviors over the multiple thermo-cycles.
To functionalize the surface of WxVO2 with thiol groups for the free radical polymerization of PNIPAm, WxVO2 nanoparticles were first coated with a silica layer. We used a literature procedure21 except that we directly used MPTMS to form the silica shell with the thiol groups instead of using a conventional alkyl siloxane followed by MPTMS coating. In the process, PVP was first dissolved in ethanol and then mixed with an ethanol dispersion of WxVO2 nanoparticles, after which the resulting mixture solution was stirred for 72 h to stabilize the nanoparticles via the adsorption of PVP chains on the nanoparticle surfaces.22,23 Following this, the formation of the silica shell using MPTMS was carried out by adding it and an ammonium solution to hydrolyze and condense the siloxanes, which was followed by stirring for 12 h.24-26 Gray nanoparticles were collected and the successful introduction of surface thiol groups was confirmed by measuring an FTIR spectrum and a TEM image of the nanoparticles. As shown in Figure 3, the FTIR spectrum of WxVO2 nanoparticles coated with a silica layer containing thiol groups show characteristic bands of C-H stretching of aliphatic alkyls (3000-2800 cm-1), S-H stretching (2560.5 cm-1), -CH2- bending (1407.8 cm-1), Si-C bending (1258.8 cm-1), and Si-O-Si asymmetric stretching (1092.5 & 1001.4 cm-1).27 The splitting of peaks at 1092.5 and 1001.4 cm-1 originates from the longitudinal and transverse optical vibrational modes of Si-O-Si asymmetric stretching vibration.28 These very strong split peaks characteristic of Si-O-Si bonds suggest that the silica skeleton formed by hydrolysis and condensation of MPTMS on the surfaces of the WxVO2 nanoparticles was sufficiently ordered. Furthermore, the TEM image shown in Figure 4(a) clearly demonstrates the shell layer of the WxVO2 nanoparticle with a thickness of 40-50 nm, further confirming the successful synthesis of the silica shell. In addition, C-H and S-H stretching bands in the FTIR spectrum suggest the existence of thiol groups in the silica shell that are available for further surface reactions.
Preparation of hybrid nanoparticles of WxVO2 and PNIPAm was carried out by polymerizing PNIPAm on the surfaces of the WxVO2 nanoparticles. NIPAm was polymerized via a free radical mechanism by utilizing a water-soluble azo-based radical initiator (VA044) and thiol groups as reacting sites on the surfaces of the WxVO2 nanoparticles.29 The successful polymerization of PNIPAm on the surfaces of the WxVO2@MPTMS nanoparticles was confirmed via measurements on an FTIR spectrum (Figure 3) and a TEM image (Figure 4(b)). The FTIR spectrum of WxVO2@MPTMS@PNIPAm shows that the S-H peak at 2560.5 cm-1 disappears and that characteristic bands for PNIPAm,30 including O-H stretching (~3433.2 cm-1), NH stretching (3280.3 cm-1), and C(=O)-N-H (1639.7 cm-1, amide I; 1540.4 cm-1, amide II), clearly appear. Moreover, PNIPAm on the WxVO2 nanoparticles was clearly observed as a light shell with a thickness of 100-200 nm surrounding the nanoparticles due to its low electron density in comparison to WxVO2, as shown in Figure 4(b) and 4(c). All of these results confirm that the hybrid structure of a WxVO2 core and a PNIPAm shell was successfully synthesized.
The compositions of these organic-inorganic hybrids were analyzed by measuring TGA pyrograms (Figure 5). Three distinct weight loss at around 100, 400 and 500 °C are allocated to the moisture evaporation, degradation of PNIPAm and inorganic components, respectively. We prepared three hybrid nanoparticles by using three different amounts of WxVO2(M) particles (0.02, 0.05, and 0.1 g) while the amount of NIPAm was fixed at 1.13 g, which means 1.7, 4.4, and 8.8 wt% of WxVO2 to PNIPAm in the hybrid nanoparticles. The residual weight percentages were -4.5, 2.9 and 7.2 wt%, respectively. Although the results show experimental errors, it was confirmed that WxVO2 nanoparticles were increasingly incorporated as designed.
The thermochromicity of the hybrid nanoparticles was examined in the wavelength range of 300-2500 nm using UV-Vis-spectroscopy below and above the phase-transition temperatures of WxVO2 and PNIPAm (20 and 40 °C, respectively). The hybrid nanoparticles were dispersed in DI water at 0.5 wt%; the resulting solutions are denoted as dispersions I, II, and III in the order of increasing WxVO2 weight percentage. The transmittance spectra of the three dispersion amounts measured at 20 and 40 °C were measured (Figure 6) and used to estimate their solar modulation (Tsol), solar modulation difference (ΔTsol), and luminous transmittance (Tlum).
Tsol at each temperature was calculated as
where φ(λ) and Tr(λ) are the solar irradiation spectrum for an air mass of 1.5, corresponding to the sun standing 37° above the horizontal31 and transmittance at specific wavelength λ, respectively.
ΔTsol was estimated by applying32
where φlum(λ) is the spectral sensitivity of the light-adapted eye.33
The estimated ΔTsol values utilizing the spectral variation depending on the temperature in Figure 5 were 5.4%, 13.9%, and 25.5% for the dispersions I, II, and III, respectively. These values show that the solar modulation ability of the hybrid nanoparticles was enhanced with increasing WxVO2 content. It is notable that the solar modulation of dispersion I with the lowest WxVO2 content can be mainly ascribed to spectral variations in the visible light range (Figure 6(a)) while the spectral variations in both the visible and NIR regions contribute to the enhanced ΔTsol values of dispersions II and III (Figure 6(b) and 6(c)). Thus, it seems that WxVO2 at over 8 wt% in the hybrid nanoparticles with PNIPAm is necessary to observe a distinct contribution from the WxVO2 toward the thermochromicity over the visible-to-NIR range.
On the other hand, the Tlum values of dispersions I, II, and III at 20 °C decreased with increasing WxVO2 content (84.8, 42.4, and 14.7%, respectively). It should be mentioned that even though the ΔTsol value of dispersion III is the highest, its Tlum value of dispersion III is too low for practical applications. This can be ascribed to the size of hybrid nanoparticles being over a few hundred nanometers and their aggregation, which can severely scatter light and decrease light transmission. It should be mentioned that the polymerization of PNIPAm via the free radical mechanism can occur both in the medium and on the surfaces of WxVO2 nanoparticles. As a result, the particles can be even connected by PNIPAm chains as shown in the TEM image (Figure 4(c)). We presume that these agglomeration of the WxVO2 nanoparticles and PNIPAm chains cause such a severe light scattering in the NIR region and the non-linear dependence of the NIR transmittance on the weight fraction of WxVO2 nanoparticles. However, our results suggest that the hybrid nanoparticles based on WxVO2 and PNIPAm with a particle size of less than 100 nm can present excellent thermochromicity in both the visible and NIR ranges.
Figure 1. XRD patterns for VO2 and WxVO2 (2 mol% W/V).
Figure 2. (a) DSC thermograms of VO2 and WxVO2 (2 mol% W/ V); (b) that at the third thermo-cycle.
FTIR spectra of WxVO2 coated with a silica layer using MPTMS and with surface-polymerized PNIPAm (WxVO2@ MPTMS@PNIPAm)
TEM images of (a) WxVO2@MPTMS; (b) WxVO2@ MPTMS@PNIPAm at a high magnification; (c) at a low magnification.
TGA pyrograms of WxVO2@MPTMS@PNIPAm with 0.02, 0.05, and 0.1 g of WxVO2.
UV-Vis spectroscopy of WxVO2@MPTMS@PNIPAm with (a) 0.02 g of WxVO2; (b) 0.05 g of WxVO2; (c) 0.1 g of WxVO2; (d) optical images of WxVO2 dispersions in aqueous media
In this study, we synthesized hybrid nanoparticles of tungsten-doped VO2 core with a PNIPAm shell that had efficient thermochromicity for smart window applications over a broad spectral range (from the visible to the NIR region). Unlike conventional gel- or film-coating type thermochromic devices, our hybrid nanoparticles were designed to be applied as an aqueous dispersion that could simply be injected in between glass plates to simply achieve low-cost smart windows. WxVO2 nanoparticles were coated with a silica shell offering numerous thiol groups via the hydrolysis and condensation of MPTMS and finally, an outermost shell of PNIPAm synthesized via free radical polymerization utilizing the surface thiol groups. The resulting aqueous dispersions of the hybrid nanoparticles presented increasing ΔTsol until the WxVO2 content reached 25%. However, it was noted that Tlum was drastically decreased down below 20% with the increase in WxVO2 in the hybrid nanoparticles, thereby making them unsuitable for practical applications. These issues suggest that further investigation should be carried out to determine the best route for synthesizing ultrafine tungsten-doped VO2 with a diameter of less than 100 nm via a process that avoids aggregation. We believe that a novel synthetic route such as a modified polyol process34 can provide WxVO2 nanoparticles with an MIT temperature at around RT and with a size less than 100 nm, enabling the formation of hybrids with negligible light scattering in the NIR region.
2021; 45(2): 286-293
Published online Mar 25, 2021
Department of Intelligent Energy and Industry, School of Chemical Engineering and Materials Science, Institute of Energy-Converting Soft Materials, Chung-Ang University, Seoul 06974, Korea