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Ag NRs hybrid hydrogel with atmospheric window full-wavelength thermal management for smart windows
Solar Energy Materials & Solar Cells 206 (2020) 110336
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Thermal-responsive PNIPAm-acrylic/Ag NRs hybrid hydrogel with atmospheric window full-wavelength thermal management for smart windows Gengyao Wei a, Dongya Yang a, Tao Zhang a, b, *, Xuejie Yue a, Fengxian Qiu a, b, ** a b
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, Jiangsu Province, China Institute of Green Chemistry and Chemical Technology, School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, Jiangsu Province, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Hybrid hydrogel Smart window Atmospheric window Thermal stimulation
Smart windows with reversible regulation on solar radiation by altering their optical transmittance in response to thermal stimuli have been developed as a promising solution toward reducing energy consumption of buildings. This work presents a new strategy for doping Ag nanorods (NRs) with poly(N-isopropylacrylamide) (PNIPAm) hydrogel to achieve atmospheric window full-wavelength thermal management. PNIPAm as basic material was aimed to achieve solar radiation modulation among visible, near infrared (NIR), and middle infrared region. Acrylic was grafted on PNIPAm chains to enhance mechanical strength. While Ag NRs were the major factors that affected solar radiation modulation due to its structural changes driven by PNIPAm phase transition. The hybrid hydrogel showed a relatively high solar modulation ability (ΔTsol) of 59.24% while maintaining luminous transmittance (Tlum) of 61.36% under room temperature. The infrared emissivity kept a little increase from 0.947 to 0.958 during phase transition and caused a 12.7 � C reduction in temperature observed from the waterwarming experiment. The innovative combination of Ag NRs and hydrogel indicated that metal nanorods may make positive effects when combining with hydrogels. This kind of new strategy based on atmospheric window full-wavelength thermal management exhibits to be an ideal candidate for applications in smart windows.
1. Introduction
violates the original intention of energy saving [12]. Comparatively, thermochromic windows with simplified structures exhibit passive strategies and self-regulating ability in response to temperature varia tion [13]. This automatic response to temperature cuts down the need for switch systems and holds promise for industrial production and ease of implementation [14]. Among varies kinds of thermochromic mate rials, hydrogels are the best-known candidate for smart windows, as they have reversible transition behavior from a transparent state to an opaque state when the temperature changes above the lower critical solution temperature (LCST) [15]. The mechanism of hydrogel from transparent to opaque can be explained in Scheme 1: at low tempera tures (below LCST), there exist intermolecular hydrogen bonds between polymer chains and surrounding water molecules, exhibiting trans parent behavior; once the temperature rises above the LCST, intermo lecular hydrogen bonds are broken and intramolecular hydrogen bonds – O and N–H groups are formed inside the polymer chains between C– [16], resulting in phase separation and polymer aggregation [17], hence
The energy consumed in the room temperature regulation occupies over 50% of building energy consumption [1,2], from which results in a large amount of carbon dioxide emissions contribute to global warming [3,4]. Smart windows that respond to external changes and further modulate solar energy entry could help save energy for indoor heating, cooling, and electric lighting [5]. Among various types of smart win dows, electrochromic materials [6,7] and thermochromic materials [8] have been reported as two of the most promising coatings with revers ibility on tuning the transmittance of ultraviolet, visible and infrared solar radiation in response to switchable extra stimuli [9,10]. Electro chromic windows are mainly electric devices with fast response speed and act as an active response to the variation of indoor temperature and light conditions [11]. However, the high cost and complex structure of electric devices limit its development, and additional energy require ment for triggering mechanism of solar radiation modulation also
* Corresponding author. School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, Jiangsu Province, China. ** Corresponding author. School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, Jiangsu Province, China. E-mail addresses: [email protected] (T. Zhang), [email protected] (F. Qiu). https://doi.org/10.1016/j.solmat.2019.110336 Received 18 August 2019; Received in revised form 19 November 2019; Accepted 2 December 2019 Available online 17 December 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.
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reducing the transparency of the hydrogel by scattering incident light [18]. Poly(N-isopropylacrylamide) (PNIPAm), a typical thermosensitive polymeric material with a porous network structure, features an LCST at around 32 � C which meets the human body’s comfort, exhibits to be a favorable candidate for smart windows [19]. PNIPAm behaves a high luminous transmittance (Tlum) at about 80% when its temperature is below LCST and shows a high solar energy modulation (ΔTsol) during its reversible phase transition. By taking advantage of this, Watanabe pre sented a large area of 1 m2 affinity intelligent window sandwiched with pure PNIPAm hydrogel showing a high luminous transmittance of 80% [20]. However, pure PNIPAm hydrogels are fragile and hard to form a homogeneous coating. To solve this problem, recent approaches have mainly focused on the combination of PNIPAm and cellulose to enhance its toughness [21–23]. Unfortunately, to some extent, the high infrared absorbing functional groups present in cellulose may interfere with solar modulation capabilities and lose their advantages. Aiming these diffi culties, Li et al. proposed a novel way of combining PNIPAm with ester – Oorganics to form thermochromic microgels [24]. Esters with just –C– and –OH bonds hardly affect solar modulation and these microparticles could achieve high luminous transmittance of 87.2% and unprecedented infrared transmittance modulation of 75.6%. The high solar trans mittance modulation could be attributed to the tunable scattering be haviors induced by organic structures, thus indicating the introduction of organic monomers with the ability to tailor the particle size and in ternal structure may be a promising way to improve thermal manage ment behavior. In order to further improve the infrared modulation ability, several groups have made great efforts on hybrids of PNIPAm hydrogel with inorganic particles. Long and co-workers introduced VO2 into PNIPAm hybrid film which shows high luminous transmittance at both room temperature and elevated temperature [25]. The introduction of VO2 has a significant impact on improving near infrared transmittance modulation due to its monoclinic-to-rutile phase transition. However, most research groups just focused on solar modulation in the visible and
NIR regions, ignoring other bands in the atmospheric window which allows solar energy passing through. Besides above regions, there exists a transparency window for electromagnetic radiation at wavelengths from 8 to 14 μm which are so-called thermal atmospheric windows [26]. Thermal atmospheric windows are absorption bandgaps with high ra diation transmission, allowing infrared energy getting through. It also coincides with the peak thermal radiation wavelengths at typical ambient temperatures [27]. Passive solar radiation can be effectively modulated by adjusting the infrared emissivity of a particular wave length. Zhai et al. intercalated resonant polar dielectric microspheres in a polymer matrix to produce a metamaterial that is completely trans parent to the solar spectrum, while having an infrared emissivity above 0.93 at the atmospheric window [28]. The metamaterial showed a sig nificant radiative cooling effect under direct solar illumination while covering a silver coating. Recently, Yue and co-workers presented a Janus thermal management membrane combined with ZnO nanorods array-coated cellulose, MnO2 nanowires, and silver nanowires as trilayer structure [29]. The ZnO nanorods array-coated cellulose exhibited high infrared emissivity to enhance heat dissipation on one side while a smooth silver nanowire layer showed low infrared emissivity for reducing heat radiation loss on the other side. The above examples revealed that inorganic particles could improve the infrared modulation ability and enhance passive radiation cooling. Herein, this work presents a sandwich-structure PNIPAm-acrylic/Ag NRs hybrid hydrogel with atmospheric window full-wavelength thermal management for smart windows. The PNIPAm-acrylic/Ag NRs structure with a stand-up transition showed a new inorganic-organic assembly method for smart windows. PNIPAm hydrogel was synthesized by free radical polymerization procedure, followed by injecting additional acrylic to enhance toughness. Homogeneous Ag NRs were prepared via a solvothermal method under relatively low temperature and mixed with PNIPAm hydrogel during the polymerization process. In this paper, temperature-responsive PNIPAm hydrogel aimed to modulate fullwavelength radiation transmittance due to its translucent-to-opaque phase transition. Grafting acrylic could enhance the roughness and
Scheme 1. Schematic of transmittance modulation based on the tunable scattering behaviors of hydrogel particles. 2
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smooth the surface of the hydrogel. The intercalation of Ag NRs in the hydrogel is intended to provide a higher intensity of control on solar modulation. The array changes of Ag NRs induced by structural changes in PNIPAm hydrogel during phase transition may lead to the variation of solar transmissivity and infrared emissivity. The combination of inor ganic nanoparticles and organic polymer leads to an atmospheric win dow full-wavelength thermal management and a hopeful way for applications in smart windows.
2.4. Characterization The morphology of Ag NRs and PNIPAm hydrogel were investigated by scanning electron microscope (SEM, JSM-6010 PLUS/LA, Tokyo, Japan). X-Ray diffraction spectroscopy (XRD-6100) was used to identify the crystal structures of Ag NRs. The mechanical property was tested by a compression deformation way with hydrogel (4 � 4 cm) sandwiched between two glasses (6 � 6 cm) and pressure resistance was measured to represent the mechanical property. The Fourier-transformed infrared (FT-IR) spectrum for a pure PNIPAm hydrogel and a PNIPAm hydrogel with acrylic were recorded using a Thermo Nicolet spectrometer (NEXUS, TM). The phase transition of resulting products was tested by differential scanning calorimetry (DSC, STA449C, NETZSCH) in nitro gen flow over the temperature range of 25–45 � C with a heating rate of 2 � C/min. The hydrogels applied for DSC testament were kept the original state and with an original polymer concentration of (5.38%). The transmittance spectra at normal incidence irradiation from 200 to 2500 nm were monitored via UV-VIS-NIR spectrometer (Shimadzu UV-3600, Shimadzu, Japan). The calculations of integral luminous transmittance Tlum (380–780 nm), NIR transmittance TNIR (780–2500 nm), solar transmittance Tsol (200–2500), and corresponding transmittance mod ulations can be calculated from Equations (1) and (2), respectively [24]: R φlum=NIR=sol ðλÞTðλÞdλ Tlum=NIR=sol ¼ R 1 φlum=NIR=sol ðλÞdλ
2. Experimental section 2.1. Materials Several chemicals of Poly(vinyl pyrrolidone) (PVP, K30), silver ni trate (AgNO3), sodium chloride (NaCl), ethylene glycol (EG), ethanol, Nisopropylacrylamide monomer (NIPAm), N,N0 -methylenebis (acryl amide) (BIS) crosslinker, potassium persulfate (KPS) initiator and N,N, N0 ,N’-tetramethylethylenediamine (TEMED) accelerator were pur chased from Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China). Acrylic (CM-205) was obtained from Zhenjiang Qimei Chemical Co., Ltd. and used as received without further purification. The quartz glass was supplied by Ai Pu optical quartz Co., Ltd, and washed several times by deionized water and ethanol, followed by drying in air. 2.2. Preparation of Ag nanorods
ΔTlum=NIR=sol ¼ Tlum=NIR=sol ðat low temperature Þ
Ag nanorods were prepared via a modified solvothermal method according to the procedure reported previously [30]. In a typical pro cedure, 1 g of PVP as a dispersant and capping agent was dissolved in 15 mL of ethylene glycol with continuous stirring and heating. Due to the greater viscosity of ethylene glycol and PVP, 0.225 g of AgNO3 was individually dissolved in 15 mL of ethylene glycol under ambient tem perature. Then the two solutions were mixed evenly and followed by adding 0.3 mL of NaCl/ethylene glycol solution (0.032 mol/L). The mixture was transferred into an 80 mL Teflon-lined autoclave and heated at 160 � C for 90 min. After the reaction, the autoclave was cooled down to room temperature and the products of Ag NRs diluted with acetone 5 times were centrifuged at 3500 rpm/10 min to remove redundant solvent and other impurities.
Tlum=NIR=sol ðat high temperature Þ
where T(λ) denotes the recorded transmittance at a selected wavelength, ϕlum is the standard luminous efficiency function for the photonic vision of human eyes, ϕNIR/sol is the NIR/solar irradiance spectrum for air mass 1.5. The infrared emissivity in the atmospheric window (8–14 μm) was measured by the IR-2 dual-band infrared emissivity measuring instru ment (Shanghai Wangjia optical instrument Co., Ltd., China). The thermal management testament was carried out by a water-warming experiment. 10 mL of water-filled centrifuge tubes with the same spec ification are placed in a model chamber. 2 cm � 2 cm of window devices made by bare double-glass slides or hydrogel devices were assembled upon the model chamber with a heat-insulating polyfoam as a middle layer. A 150 W infrared lamp was set 25 cm above the device and the heat from the infrared lamp passed through the window and heated deionized water in the tube underneath. The characterizations were carried out under the room condition of 27 � C and infrared thermal images were recorded by an infrared thermal camera (FLIR ONE PRO, FLIR Systems, Inc., OR, USA) and analyzed by accessory software in real time.
2.3. Preparation of PNIPAm hydrogels The cross-linked PNIPAm hydrogel was prepared via a free radical polymerization procedure [31]. First, 0.428 g of NIPAm, 0.01 g of BIS and 0.02 g of KPS were dissolved in 8 mL of deionized water, followed by adding 50 μL of acrylic and 50 μL of Ag NRs solution. Then the mixture was transferred into a sandwiched glass mold and placed under a ni trogen atmosphere to remove any traces of residual oxygen. Finally, 10 μL of TEMED was added into the solution to initiate the reaction. The gelation process took 2 h under room temperature and then purified by deionized water to remove unreacted chemicals. In order to investigate the effect of Ag NRs concentration on the performance of solar modu lation ability, different amounts of Ag NRs were selectively added and different parameters are listed in Table 1.
3. Results and discussion 3.1. Preparation of Ag NRs Ag NRs was synthesized via solvothermal method under relatively lower temperature. The solvothermal method has the advantages of simplicity and energy saving compared to the polyol process which depends on a higher temperature. Fig. 1a shows straight Ag NRs with a homogeneous diameter of 40–50 nm and an average length of 1–2 μm. There are no other impurities appearing in the devices which could also be proved by the XRD pattern (Fig. 1b). As a dispersant and a capping agent, PVP can selectively passivate on (100) lattice of Ag seeds and allows longitudinal growth of the (111) lattice [32]. Other peaks of (200), (220) and (331) are also indexed to the fcc structure of Ag NRs. No other peaks exist in the picture, further confirming the high purity of as-prepared Ag NRs.
Table 1 Component of hybrid hydrogels. Sample
NIPAm
BIS
KPS
TEMED
Acrylic
Ag NRs
I II III IV V VI VII
0.428 0.428 0.428 0.428 0.428 0.428 0.428
0.01 g 0.01 g 0.01 g 0.01 g 0.01 g 0.01 g 0.01 g
0.02 g 0.02 g 0.02 g 0.02 g 0.02 g 0.02 g 0.02 g
10 10 10 10 10 10 10
/ 50 μL 50 μL 50 μL 25 μL 75 μL 100 μL
/ / 0.1 mg 1 mg / / /
g g g g g g g
μL μL μL μL μL μL μL
2
3
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Fig. 1. (a) SEM image of Ag NRs; (b) XRD pattern of Ag NRs.
3.2. Preparation of PNIPAm hydrogels
acrylic improves mechanical properties while maintaining light trans mission and hardly increasing infrared absorption. When the acrylic solution was added into the precursor, Ag NRs with different concen trations were also dropped into the mixed solution to form hybrid hydrogels. Fig. 2e and f shows PNIPAm-acrylic/Ag NRs hydrogel with 0.23 wt% of Ag NRs. It can be clearly seen from the images that Ag NRs were coated by polymer and pointed to different directions. These polymer-coated Ag NRs covered on hydrogel trunks and formed flower-like structure. In response to the temperature stimuli, polymer would guide the open or close of the “flower” and the structural changes may lead to a dramatic variation in infrared emissivity. In order to prove the influence of different amounts of acrylic on mechanical properties of hydrogels, a pressure resistance test was carried out and results were shown in Fig. 2d. It can be seen that pure hydrogel (SI) can just resist 450 g weight. While adding acrylic, the pressure resistance rose with the increase of the concentration of acrylic. This result could prove that acrylic can help hydrogel improve its mechanical property.
PNIPAm hydrogels with different amounts of Ag NRs were synthe sized via a free radical polymerization and their morphologies were characterized by scanning electron microscope after freeze drying. As shown in Fig. 2a, pure PNIPAm hydrogel surface exhibits a porous network structure which is woven by a polymer [33]. The formation of network structure may be ascribed to the occupation of water during the polymerization process. And such porous structure allows the connec tion or disconnection of hydrogen bond between water and polymer. When temperature gets changed, alteration in hydrogen bonding affects the stretching or contraction of the net, which in turn leads to changes in light transmission. However, when meeting squeezing or stretching, porous network structure may be easily torn (Fig. 2c). In order to enhance the firmness, small amount of acrylic solution was added into the PINPAm hydrogel and its morphology is shown in Fig. 2b. After freeze-drying process, PNIPAm-acrylic hydrogel exhibits root-like structure with wrinkles spreading on its surface. The stripe-like wrin kles may be attributed to the integration of acrylic and PNIPAm polymer bonds, which enhances stretch resistance (Fig. 2c). The addition of
Fig. 2. SEM of PNIPAm hydrogel (a), PNIPAm-acrylic hydrogel (b), PNIPAm-acrylic/Ag NRs hydrogel (e) (f); schematic of PNIPAm and PNIPAm-acrylic hydrogel under stretch condition (c); pressure resistance of different samples (d). 4
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3.3. FT-IR spectra To find out the function group and molecular interaction of PNIPAm, PNIPAm-acrylic and PNIPAm-acrylic/Ag NRs with different concentra tions, FTIR spectra were employed and results are shown in Fig. 3a. As for sample I (pure PNIPAm hydrogel), the overlapping double peaks at 3435 and 3296 cm 1 can be ascribed to asymmetrical and symmetrical stretching vibrations of N–H, respectively. The characteristic absorp tions of amide-I and II bands are observed at 1646 and 1540 cm 1. While for samples II, III, and IV, there appears a weak peak at 1732 cm 1, – O absorption peak, thus confirming acrylic which attributes to the C– has already grafted on PNIPAm hydrogel. As the metal element does not have IR absorption, it shows no characteristic peaks of Ag NRs. 3.4. Differential scanning calorimetry DSC heating thermograms were used to investigate the effects of acrylic and Ag NRs content on the LCST of four samples (pure PNIPAm hydrogel and hydrogels with acrylic and different concentration of Ag NRs), results were recorded in Fig. 3b. It can be seen from the spectra that pure hydrogel (SI) showed LCST of 31.4 � C, while the peak positions of other samples all remain at 32.9 � C. This indicates that addition of acrylic may increase the LCST of hydrogel. It has been reported that the LCST depends on the ratio of hydrophilic (–CONH–) and hydrophobic (-CH(CH3)2) groups on the PNIPAm chains [34,35]. In this work, the adjunction of Ag NRs during the in-situ polymerization does not alter the ratio of both groups in the PNIPAm so that the resultant samples show the almost same LCST compared to each other. Furthermore, LCST of 32.9 � C is an appropriate temperature to distinguish between hot or cool outdoors, indicating that the hybrid hydrogel in this work is a suitable candidate for smart window devices on regulating indoor temperature.
Fig. 4. UV–visible–NIR transmittance spectra of four samples at 25 � C and 40 � C (The inset is the solar irradiance spectrum, filled area).
NRs hardly affected the light transmission at room temperature. When the temperature raised to 40 � C, all samples behaved transparent-toopaque transition and their transmittance has declined to varying de grees. The device with pure PNIPAm showed decreased transmittance of 48.58% in the visible region and its ΔTsol keeps at 26.88% (Table 2). Compared with sample I, PNIPAm hydrogel with acrylic has a much higher transmittance variation rate at 40 � C both in visible region and NIR region. When adding Ag NRs, it could be found that doping of Ag NRs makes a great deal on transmittance modulation and the concen tration of Ag NRs is a major factor affecting the transmittance. Ag NRs were initially attached to the surface of hydrogel. While the comparison of Fig. 2b and f showed a shrinkage deformation after thermal drying, the deformation of hydrogel caused the variation of Ag NRs. As tem perature rises above LCST, the hydrogel structure shrinks and deforms, leading Ag NRs to a stand-up variation (Scheme 2). This change enhances light scattering and increases solar modulation ability. Various Ag NRs concentrations lead to different light scattering abilities, resulting in variation on transmittance modulation capabilities. Comparing the three samples (II, III, IV) in Table 2, the transmittance modulation of each sample containing different concentrations of Ag NRs differed by about 10% in both the visible region and the NIR region. Further con firming Ag NRs could help promote transmittance modulation. Fig. 5a depicts solar light transmittance (200–2500 nm) profiles of PNIPAm-acrylic/Ag NRs hydrogel (sample IV) under various tempera tures. The two prominent peaks observed at 1450 and 1932 nm result from the hydrogen bond due to water trapped in hydrogel [36]. The absorption peak of water at 1450 nm is attributed to the O–H stretch in the water molecule, and the other peak at 1932 nm is due to a
3.5. Transmittance & modulation effect To investigate the thermal sensitivity properties, samples were sandwiched in two glass sheets as smart window devices. The trans mittance spectra of four samples at a wavelength of 200–2500 nm under 25 and 40 � C are reported in Fig. 4 and integral calculation results are summarized in Table 2. It can be clearly seen that sandwich-structure devices with pure PNIPAm showed a relatively high transmittance of 73.20% in the visible region compared with the other three samples under 25 � C. While the transmittance of the other three samples dis played around 62% at wavelength between 380 and 780 nm, and it also decreases significantly after phase transition. This may be attributed to the grafting of acrylic that influenced the refraction and absorption of light. Moreover, samples (II, III, IV) with different concentrations of Ag NRs showed close transmittance values, thus indicating the doping of Ag
Fig. 3. (a) FT-IR of four samples; (b) DSC of four samples. 5
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Table 2 Transmittance and transmittance modulation of four samples in the visible and NIR region. T ¼ 25 � C SI SII SIII SIV
T ¼ 40 � C
Tranasmitance modulation
Tlum
TNIR
Tsol
Tlum
TNIR
Tsol
ΔTlum
ΔTNIR
ΔTsol
73.20 62.57 62.07 61.36
76.83 67.49 67.39 65.85
74.75 64.69 64.36 63.29
48.58 20.04 12.99 3.43
46.82 28.79 15.30 4.85
47.87 23.83 13.99 4.05
24.62 42.53 49.08 57.93
30.01 38.70 52.09 61.00
26.88 40.86 50.37 59.24
combination of O–H stretch and H–O–H bending. When the temperature is lower than LCST, much more hydrogen bonds exist between water and PNIPAm polymer chains; as temperature goes up, the hydrogen bonds begin to break up and cause continuous phase separation. The oscillator strength of the O–H bond is proportional to the hydrogen bond energy; therefore, the intensity of the transmittance valley decreases in case where the existing hydrogen bonds become fewer [5]. As for PNIPAm-acrylic/Ag NRs, it maintained transparency below the LCST with the calculated Tlum of 61.36% at 25 � C and slightly reduced Tlum of 60.33% at 28 � C (Fig. 5b). While its Tlum dropped down to 41.55% when the temperature rose to 31 � C and further declined to 3.79% at 34 � C, suggesting the LCST occurred between 31 � C and 34 � C, in line with DSC measurement. Later the transmittance decreased slowly and both ΔTlum and ΔTsol reached plateau afterward with temperature increase. As a result, Tlum was significantly reduced from 61.36% to 3.43%, and ΔTsol was boosted to 59.24% (Fig. 5b). Such a dramatic transition is desired for promptly blocking a large amount of solar irradiance especially in
Scheme 2. Schematic of enhanced solar modulation based on a stand-up vari ation of Ag nanorods.
Fig. 5. (a) UV–visible–NIR transmittance spectra of SIV at different temperatures (The inset is the solar irradiance spectrum, filled area); (b) The luminous (Tlum), NIR (TNIR), and solar (Tsol) transmittances (solid lines) of SIV and corresponding transmittance modulations (ΔTlum, ΔTNIR, ΔTsol) (dashed lines) in response to the temperature.
Fig. 6. (a) Luminous transmittance (Tlum) at 25 � C and 40 � C, and solar transmittance modulation (ΔTsol) of SIV during 20 cycles; (b) images of smart window device that consists of SIV after 20 cycles at 25 � C and 40 � C. 6
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the visible and NIR region [37]. As in practical field applications, it is vitally important for smart windows to achieve a reversible optical transition. Hence, the reversible and repeatable optical switching properties of PNIPAm-acrylic/Ag NRs hydrogel (sample SIV) were investigated and results were shown in Fig. 6a. After 20 cycles, the Tlum at both high and low temperatures are consistent and ΔTsol remains unchanged. This indicates that both transmittance and solar modulation ability of such hybrid device should be reliable in an actual smart window application as long as good sealing to prevent water evaporation is achieved. Images of smart window de vice with SIV hydrogel were photoed at 25 � C and 40 � C after 20 cycles’ phase transition. The hydrogel exhibited excellent optical stability while maintaining clear light transmission.
reached 44 � C (16.9 � C increments), whereas device with SIV exhibited much lower increase rate about 0.61 � C/min and finally reached 36.3 � C (9.2 � C increments). This can be attributed to the solar energy shielding caused by phase transition of PNIPAm hydrogel and transmittance modulation influenced by Ag NRs structural changes. Unexpectedly, it appears a slight temperature decline after 6 min both in two hydrogel samples, which may indicate a net negative heat gain inside the tube induced by a sudden opacity change whereby the heat dissipation is more significant than the heat input from illumination. After 15 min illumination, temperatures of water under three devices all reached plateau and gradually dropped down to a similar value. Based on above experiments, PNIPAm-acrylic/Ag NRs hydrogel behaves an effective way on modulating solar energy and thus to be an excellent material for smart windows.
3.6. Infrared emissivity
3.8. Comparisons of Tlum and ΔTsol
To investigate the changes of infrared emissivity of hydrogel under different temperature conditions, infrared parameters of four samples were measured by IR-2 Dual-Band Infrared Emissivity Measuring In strument and the results were shown in Table 3. Four samples all showed high infrared emissivity above 0.9 both in the thermal infrared region (8–14 μm) and middle infrared region (1–22 μm). This may be attributed to the strong absorption of organic functional groups for infrared ther mal radiation. However, strong absorption capacity is equivalent to high heat dissipation, which is also effective in radiative cooling under thermal conditions. For pure PNIPAm hydrogel (SI), its infrared emis sivity is the highest among four samples, this is due to the rough surface caused by its porous network structure. Later when temperature goes up to 40 � C, its surface became smooth after phase transition and infrared emissivity value dropdown. Comparing the other three samples, hydrogels with acrylic exhibit smooth surface and relatively lower values. When the temperature exceeds the LCST, the emissivity value of each sample increases to a different extent, which is beneficial to ach ieve radiative heat dissipation at high temperatures. It can also be found that as the Ag NRs content increases, the value of infrared emissivity decreases and emissivity modulation becomes large. This indicates Ag NRs also have modulation ability on thermal management in the infrared region.
Fig. 8 displays a comparison of our work with recently reported smart window devices based on VO2 and hydrogel composites. As unilaterally pursuing superior solar modulation capability or ultrahigh luminous transmittance is meaningless, it is vitally important to consider high Tlum accompanied by high ΔTsol. For VO2 composites, both Tlum and ΔTsol all stayed at low level which is not suitable for applying in the smart windows. While after VO2 particles combining with ion liquid or hydrogel, the ΔTsol could reach about 30–35%. Interestingly, Lee and his group present a thermochromic ionogel which exhibits Tlum of 88% and ΔTsol of 57% [38]. However, these research all lack concerns of infrared emissivity in the thermal infrared region. In such a case, the current PNIPAm-acrylic/Ag NRs hydrogel with Tlum of 61.36% accompanied by ΔTsol of 59.24% and infrared modulation ability is extremely competi tive for the smart windows. 4. Conclusions In summary, a new type of full wavelength thermal management window device based on PNIPAm-acrylic/Ag NRs hydrogel has been investigated systematically in this paper. Through a comprehensive analysis of the hydrogel optical switching behavior, the mechanism for enhanced transmittance modulation ability and emissivity regulation ability was elucidated by studying the doping concentrations of different Ag NRs and grafting acrylic. With proper acrylic and Ag NRs concen tration, hydrogel can be fully transparent at room temperature with Tlum of 61.36%, and achieving high ΔTlum of 57.93% after phase transition. Meanwhile, the hybrid hydrogel showed ΔTNIR of 61.00% in the tem perature range of 25 � C–40 � C. The aspects of both luminous and NIR shielding characteristics would achieve dual regulation mode on ther mal management, which may be beneficial on application in smart windows for a better indoor comfort environment. Moreover, the solar modulation ability of hybrid hydrogel was found to be reversible and repeatable after 20 cycles of irradiation. Apart from solar modulation in the visible region and NIR region, infrared emissivity value of hybrid hydrogel got a slight increase from 0.947 to 0.958 (1–22 μm) when temperature rises above the LCST, which is effective for accomplishing radiative cooling indoors. While water-warming experiment also sup ports the point with 12.7 � C reductions of hybrid hydrogel compared with blank glass. Such large and full-wavelength solar modulation makes the hybrid hydrogel an ideal candidate for smart windows ap plications. And the new method for inorganic-organic assembly may give an inspiration for hybrid materials based on smart windows.
3.7. Energy saving performance In order to characterize the reduction of the heat flux from solar irradiance due to the phase transition of the hydrogel, a water-warming experiment was carried out to simulate indoor thermal management and visually demonstrate the energy-saving efficiency of the hybrid hydro gel. Three window devices were installed on chambers above three tubes with equal amount of water. The blank device with two quartz glasses was set as a control experiment on the left, sandwich-structure devices filling with SIII and SIV were set on the middle and right side, respec tively. Temperature changes were recorded in Fig. 7a under 1 sun, air mass 1.5 illuminations. At first, the initial temperatures of water under three window devices were all 27.1 � C. While the temperature of water under blank glass increased rapidly to 49 � C after light on for 15 min (21.9 � C increments). As for the other two sandwich-structure devices, device filling with SIII showed low increase rate about 1.13 � C/min and Table 3 Infrared emissivity of four samples in the thermal infrared region (8–14 μm) and middle infrared region (1–22 μm). 8–14 μm SI SII SIII SIV
1–22 μm
25 � C
40 � C
25 � C
40 � C
0.986 0.968 0.959 0.958
0.965 0.972 0.962 0.968
0.965 0.961 0.953 0.947
0.95 0.966 0.961 0.958
Author contributions section Gengyao Wei: Methodology, Supervision, Writing-Original Draft. Dongya Yang: Investigation, Resources. Tao Zhang: Funding acquisi tion, Supervision, Writing-review editing. Xuejie Yue: Data curation, Validation. Fengxian Qiu: Conceptualization. 7
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Fig. 7. (a) Temperature profiles of a thermometer inside the model chamber affixed with Blank glass (black), SIII (green) or SIV (blue) as the window; (b) compared thermal images from different irradiation time. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 8. Luminous transmittance (Tlum) and solar energy modulation (ΔTsol) of different smart windows: single-layered VO2 films [39-41], VO2/SiO2 com posite films [42-44], doped VO2 films [45–49], VO2/hydrogel films [5,25], VO2/IL-Ni-Cl films [50], and PNIPAm-acrylic/Ag NRs films in this work.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This work was financially supported by National Natural Science Foundation of China (21706100 and 21878132), Natural Science Foundation of Hebei Province (B2019108017), Xingtai Polytechnic College Application Innovation Project (20190106). Youth Talent Cultivation Program of Jiangsu University, High-Level Personnel Training Project of Jiangsu Province (BRA2016142) and Key Laboratory of Pollution Control and Resource Reuse Foundation (NO. PCRRF18003). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110336. 8
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Contents lists available at ScienceDirect
Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat
Thermal-responsive PNIPAm-acrylic/Ag NRs hybrid hydrogel with atmospheric window full-wavelength thermal management for smart windows Gengyao Wei a, Dongya Yang a, Tao Zhang a, b, *, Xuejie Yue a, Fengxian Qiu a, b, ** a b
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, Jiangsu Province, China Institute of Green Chemistry and Chemical Technology, School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, Jiangsu Province, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Hybrid hydrogel Smart window Atmospheric window Thermal stimulation
Smart windows with reversible regulation on solar radiation by altering their optical transmittance in response to thermal stimuli have been developed as a promising solution toward reducing energy consumption of buildings. This work presents a new strategy for doping Ag nanorods (NRs) with poly(N-isopropylacrylamide) (PNIPAm) hydrogel to achieve atmospheric window full-wavelength thermal management. PNIPAm as basic material was aimed to achieve solar radiation modulation among visible, near infrared (NIR), and middle infrared region. Acrylic was grafted on PNIPAm chains to enhance mechanical strength. While Ag NRs were the major factors that affected solar radiation modulation due to its structural changes driven by PNIPAm phase transition. The hybrid hydrogel showed a relatively high solar modulation ability (ΔTsol) of 59.24% while maintaining luminous transmittance (Tlum) of 61.36% under room temperature. The infrared emissivity kept a little increase from 0.947 to 0.958 during phase transition and caused a 12.7 � C reduction in temperature observed from the waterwarming experiment. The innovative combination of Ag NRs and hydrogel indicated that metal nanorods may make positive effects when combining with hydrogels. This kind of new strategy based on atmospheric window full-wavelength thermal management exhibits to be an ideal candidate for applications in smart windows.
1. Introduction
violates the original intention of energy saving [12]. Comparatively, thermochromic windows with simplified structures exhibit passive strategies and self-regulating ability in response to temperature varia tion [13]. This automatic response to temperature cuts down the need for switch systems and holds promise for industrial production and ease of implementation [14]. Among varies kinds of thermochromic mate rials, hydrogels are the best-known candidate for smart windows, as they have reversible transition behavior from a transparent state to an opaque state when the temperature changes above the lower critical solution temperature (LCST) [15]. The mechanism of hydrogel from transparent to opaque can be explained in Scheme 1: at low tempera tures (below LCST), there exist intermolecular hydrogen bonds between polymer chains and surrounding water molecules, exhibiting trans parent behavior; once the temperature rises above the LCST, intermo lecular hydrogen bonds are broken and intramolecular hydrogen bonds – O and N–H groups are formed inside the polymer chains between C– [16], resulting in phase separation and polymer aggregation [17], hence
The energy consumed in the room temperature regulation occupies over 50% of building energy consumption [1,2], from which results in a large amount of carbon dioxide emissions contribute to global warming [3,4]. Smart windows that respond to external changes and further modulate solar energy entry could help save energy for indoor heating, cooling, and electric lighting [5]. Among various types of smart win dows, electrochromic materials [6,7] and thermochromic materials [8] have been reported as two of the most promising coatings with revers ibility on tuning the transmittance of ultraviolet, visible and infrared solar radiation in response to switchable extra stimuli [9,10]. Electro chromic windows are mainly electric devices with fast response speed and act as an active response to the variation of indoor temperature and light conditions [11]. However, the high cost and complex structure of electric devices limit its development, and additional energy require ment for triggering mechanism of solar radiation modulation also
* Corresponding author. School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, Jiangsu Province, China. ** Corresponding author. School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, Jiangsu Province, China. E-mail addresses: [email protected] (T. Zhang), [email protected] (F. Qiu). https://doi.org/10.1016/j.solmat.2019.110336 Received 18 August 2019; Received in revised form 19 November 2019; Accepted 2 December 2019 Available online 17 December 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.
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reducing the transparency of the hydrogel by scattering incident light [18]. Poly(N-isopropylacrylamide) (PNIPAm), a typical thermosensitive polymeric material with a porous network structure, features an LCST at around 32 � C which meets the human body’s comfort, exhibits to be a favorable candidate for smart windows [19]. PNIPAm behaves a high luminous transmittance (Tlum) at about 80% when its temperature is below LCST and shows a high solar energy modulation (ΔTsol) during its reversible phase transition. By taking advantage of this, Watanabe pre sented a large area of 1 m2 affinity intelligent window sandwiched with pure PNIPAm hydrogel showing a high luminous transmittance of 80% [20]. However, pure PNIPAm hydrogels are fragile and hard to form a homogeneous coating. To solve this problem, recent approaches have mainly focused on the combination of PNIPAm and cellulose to enhance its toughness [21–23]. Unfortunately, to some extent, the high infrared absorbing functional groups present in cellulose may interfere with solar modulation capabilities and lose their advantages. Aiming these diffi culties, Li et al. proposed a novel way of combining PNIPAm with ester – Oorganics to form thermochromic microgels [24]. Esters with just –C– and –OH bonds hardly affect solar modulation and these microparticles could achieve high luminous transmittance of 87.2% and unprecedented infrared transmittance modulation of 75.6%. The high solar trans mittance modulation could be attributed to the tunable scattering be haviors induced by organic structures, thus indicating the introduction of organic monomers with the ability to tailor the particle size and in ternal structure may be a promising way to improve thermal manage ment behavior. In order to further improve the infrared modulation ability, several groups have made great efforts on hybrids of PNIPAm hydrogel with inorganic particles. Long and co-workers introduced VO2 into PNIPAm hybrid film which shows high luminous transmittance at both room temperature and elevated temperature [25]. The introduction of VO2 has a significant impact on improving near infrared transmittance modulation due to its monoclinic-to-rutile phase transition. However, most research groups just focused on solar modulation in the visible and
NIR regions, ignoring other bands in the atmospheric window which allows solar energy passing through. Besides above regions, there exists a transparency window for electromagnetic radiation at wavelengths from 8 to 14 μm which are so-called thermal atmospheric windows [26]. Thermal atmospheric windows are absorption bandgaps with high ra diation transmission, allowing infrared energy getting through. It also coincides with the peak thermal radiation wavelengths at typical ambient temperatures [27]. Passive solar radiation can be effectively modulated by adjusting the infrared emissivity of a particular wave length. Zhai et al. intercalated resonant polar dielectric microspheres in a polymer matrix to produce a metamaterial that is completely trans parent to the solar spectrum, while having an infrared emissivity above 0.93 at the atmospheric window [28]. The metamaterial showed a sig nificant radiative cooling effect under direct solar illumination while covering a silver coating. Recently, Yue and co-workers presented a Janus thermal management membrane combined with ZnO nanorods array-coated cellulose, MnO2 nanowires, and silver nanowires as trilayer structure [29]. The ZnO nanorods array-coated cellulose exhibited high infrared emissivity to enhance heat dissipation on one side while a smooth silver nanowire layer showed low infrared emissivity for reducing heat radiation loss on the other side. The above examples revealed that inorganic particles could improve the infrared modulation ability and enhance passive radiation cooling. Herein, this work presents a sandwich-structure PNIPAm-acrylic/Ag NRs hybrid hydrogel with atmospheric window full-wavelength thermal management for smart windows. The PNIPAm-acrylic/Ag NRs structure with a stand-up transition showed a new inorganic-organic assembly method for smart windows. PNIPAm hydrogel was synthesized by free radical polymerization procedure, followed by injecting additional acrylic to enhance toughness. Homogeneous Ag NRs were prepared via a solvothermal method under relatively low temperature and mixed with PNIPAm hydrogel during the polymerization process. In this paper, temperature-responsive PNIPAm hydrogel aimed to modulate fullwavelength radiation transmittance due to its translucent-to-opaque phase transition. Grafting acrylic could enhance the roughness and
Scheme 1. Schematic of transmittance modulation based on the tunable scattering behaviors of hydrogel particles. 2
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smooth the surface of the hydrogel. The intercalation of Ag NRs in the hydrogel is intended to provide a higher intensity of control on solar modulation. The array changes of Ag NRs induced by structural changes in PNIPAm hydrogel during phase transition may lead to the variation of solar transmissivity and infrared emissivity. The combination of inor ganic nanoparticles and organic polymer leads to an atmospheric win dow full-wavelength thermal management and a hopeful way for applications in smart windows.
2.4. Characterization The morphology of Ag NRs and PNIPAm hydrogel were investigated by scanning electron microscope (SEM, JSM-6010 PLUS/LA, Tokyo, Japan). X-Ray diffraction spectroscopy (XRD-6100) was used to identify the crystal structures of Ag NRs. The mechanical property was tested by a compression deformation way with hydrogel (4 � 4 cm) sandwiched between two glasses (6 � 6 cm) and pressure resistance was measured to represent the mechanical property. The Fourier-transformed infrared (FT-IR) spectrum for a pure PNIPAm hydrogel and a PNIPAm hydrogel with acrylic were recorded using a Thermo Nicolet spectrometer (NEXUS, TM). The phase transition of resulting products was tested by differential scanning calorimetry (DSC, STA449C, NETZSCH) in nitro gen flow over the temperature range of 25–45 � C with a heating rate of 2 � C/min. The hydrogels applied for DSC testament were kept the original state and with an original polymer concentration of (5.38%). The transmittance spectra at normal incidence irradiation from 200 to 2500 nm were monitored via UV-VIS-NIR spectrometer (Shimadzu UV-3600, Shimadzu, Japan). The calculations of integral luminous transmittance Tlum (380–780 nm), NIR transmittance TNIR (780–2500 nm), solar transmittance Tsol (200–2500), and corresponding transmittance mod ulations can be calculated from Equations (1) and (2), respectively [24]: R φlum=NIR=sol ðλÞTðλÞdλ Tlum=NIR=sol ¼ R 1 φlum=NIR=sol ðλÞdλ
2. Experimental section 2.1. Materials Several chemicals of Poly(vinyl pyrrolidone) (PVP, K30), silver ni trate (AgNO3), sodium chloride (NaCl), ethylene glycol (EG), ethanol, Nisopropylacrylamide monomer (NIPAm), N,N0 -methylenebis (acryl amide) (BIS) crosslinker, potassium persulfate (KPS) initiator and N,N, N0 ,N’-tetramethylethylenediamine (TEMED) accelerator were pur chased from Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China). Acrylic (CM-205) was obtained from Zhenjiang Qimei Chemical Co., Ltd. and used as received without further purification. The quartz glass was supplied by Ai Pu optical quartz Co., Ltd, and washed several times by deionized water and ethanol, followed by drying in air. 2.2. Preparation of Ag nanorods
ΔTlum=NIR=sol ¼ Tlum=NIR=sol ðat low temperature Þ
Ag nanorods were prepared via a modified solvothermal method according to the procedure reported previously [30]. In a typical pro cedure, 1 g of PVP as a dispersant and capping agent was dissolved in 15 mL of ethylene glycol with continuous stirring and heating. Due to the greater viscosity of ethylene glycol and PVP, 0.225 g of AgNO3 was individually dissolved in 15 mL of ethylene glycol under ambient tem perature. Then the two solutions were mixed evenly and followed by adding 0.3 mL of NaCl/ethylene glycol solution (0.032 mol/L). The mixture was transferred into an 80 mL Teflon-lined autoclave and heated at 160 � C for 90 min. After the reaction, the autoclave was cooled down to room temperature and the products of Ag NRs diluted with acetone 5 times were centrifuged at 3500 rpm/10 min to remove redundant solvent and other impurities.
Tlum=NIR=sol ðat high temperature Þ
where T(λ) denotes the recorded transmittance at a selected wavelength, ϕlum is the standard luminous efficiency function for the photonic vision of human eyes, ϕNIR/sol is the NIR/solar irradiance spectrum for air mass 1.5. The infrared emissivity in the atmospheric window (8–14 μm) was measured by the IR-2 dual-band infrared emissivity measuring instru ment (Shanghai Wangjia optical instrument Co., Ltd., China). The thermal management testament was carried out by a water-warming experiment. 10 mL of water-filled centrifuge tubes with the same spec ification are placed in a model chamber. 2 cm � 2 cm of window devices made by bare double-glass slides or hydrogel devices were assembled upon the model chamber with a heat-insulating polyfoam as a middle layer. A 150 W infrared lamp was set 25 cm above the device and the heat from the infrared lamp passed through the window and heated deionized water in the tube underneath. The characterizations were carried out under the room condition of 27 � C and infrared thermal images were recorded by an infrared thermal camera (FLIR ONE PRO, FLIR Systems, Inc., OR, USA) and analyzed by accessory software in real time.
2.3. Preparation of PNIPAm hydrogels The cross-linked PNIPAm hydrogel was prepared via a free radical polymerization procedure [31]. First, 0.428 g of NIPAm, 0.01 g of BIS and 0.02 g of KPS were dissolved in 8 mL of deionized water, followed by adding 50 μL of acrylic and 50 μL of Ag NRs solution. Then the mixture was transferred into a sandwiched glass mold and placed under a ni trogen atmosphere to remove any traces of residual oxygen. Finally, 10 μL of TEMED was added into the solution to initiate the reaction. The gelation process took 2 h under room temperature and then purified by deionized water to remove unreacted chemicals. In order to investigate the effect of Ag NRs concentration on the performance of solar modu lation ability, different amounts of Ag NRs were selectively added and different parameters are listed in Table 1.
3. Results and discussion 3.1. Preparation of Ag NRs Ag NRs was synthesized via solvothermal method under relatively lower temperature. The solvothermal method has the advantages of simplicity and energy saving compared to the polyol process which depends on a higher temperature. Fig. 1a shows straight Ag NRs with a homogeneous diameter of 40–50 nm and an average length of 1–2 μm. There are no other impurities appearing in the devices which could also be proved by the XRD pattern (Fig. 1b). As a dispersant and a capping agent, PVP can selectively passivate on (100) lattice of Ag seeds and allows longitudinal growth of the (111) lattice [32]. Other peaks of (200), (220) and (331) are also indexed to the fcc structure of Ag NRs. No other peaks exist in the picture, further confirming the high purity of as-prepared Ag NRs.
Table 1 Component of hybrid hydrogels. Sample
NIPAm
BIS
KPS
TEMED
Acrylic
Ag NRs
I II III IV V VI VII
0.428 0.428 0.428 0.428 0.428 0.428 0.428
0.01 g 0.01 g 0.01 g 0.01 g 0.01 g 0.01 g 0.01 g
0.02 g 0.02 g 0.02 g 0.02 g 0.02 g 0.02 g 0.02 g
10 10 10 10 10 10 10
/ 50 μL 50 μL 50 μL 25 μL 75 μL 100 μL
/ / 0.1 mg 1 mg / / /
g g g g g g g
μL μL μL μL μL μL μL
2
3
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Fig. 1. (a) SEM image of Ag NRs; (b) XRD pattern of Ag NRs.
3.2. Preparation of PNIPAm hydrogels
acrylic improves mechanical properties while maintaining light trans mission and hardly increasing infrared absorption. When the acrylic solution was added into the precursor, Ag NRs with different concen trations were also dropped into the mixed solution to form hybrid hydrogels. Fig. 2e and f shows PNIPAm-acrylic/Ag NRs hydrogel with 0.23 wt% of Ag NRs. It can be clearly seen from the images that Ag NRs were coated by polymer and pointed to different directions. These polymer-coated Ag NRs covered on hydrogel trunks and formed flower-like structure. In response to the temperature stimuli, polymer would guide the open or close of the “flower” and the structural changes may lead to a dramatic variation in infrared emissivity. In order to prove the influence of different amounts of acrylic on mechanical properties of hydrogels, a pressure resistance test was carried out and results were shown in Fig. 2d. It can be seen that pure hydrogel (SI) can just resist 450 g weight. While adding acrylic, the pressure resistance rose with the increase of the concentration of acrylic. This result could prove that acrylic can help hydrogel improve its mechanical property.
PNIPAm hydrogels with different amounts of Ag NRs were synthe sized via a free radical polymerization and their morphologies were characterized by scanning electron microscope after freeze drying. As shown in Fig. 2a, pure PNIPAm hydrogel surface exhibits a porous network structure which is woven by a polymer [33]. The formation of network structure may be ascribed to the occupation of water during the polymerization process. And such porous structure allows the connec tion or disconnection of hydrogen bond between water and polymer. When temperature gets changed, alteration in hydrogen bonding affects the stretching or contraction of the net, which in turn leads to changes in light transmission. However, when meeting squeezing or stretching, porous network structure may be easily torn (Fig. 2c). In order to enhance the firmness, small amount of acrylic solution was added into the PINPAm hydrogel and its morphology is shown in Fig. 2b. After freeze-drying process, PNIPAm-acrylic hydrogel exhibits root-like structure with wrinkles spreading on its surface. The stripe-like wrin kles may be attributed to the integration of acrylic and PNIPAm polymer bonds, which enhances stretch resistance (Fig. 2c). The addition of
Fig. 2. SEM of PNIPAm hydrogel (a), PNIPAm-acrylic hydrogel (b), PNIPAm-acrylic/Ag NRs hydrogel (e) (f); schematic of PNIPAm and PNIPAm-acrylic hydrogel under stretch condition (c); pressure resistance of different samples (d). 4
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3.3. FT-IR spectra To find out the function group and molecular interaction of PNIPAm, PNIPAm-acrylic and PNIPAm-acrylic/Ag NRs with different concentra tions, FTIR spectra were employed and results are shown in Fig. 3a. As for sample I (pure PNIPAm hydrogel), the overlapping double peaks at 3435 and 3296 cm 1 can be ascribed to asymmetrical and symmetrical stretching vibrations of N–H, respectively. The characteristic absorp tions of amide-I and II bands are observed at 1646 and 1540 cm 1. While for samples II, III, and IV, there appears a weak peak at 1732 cm 1, – O absorption peak, thus confirming acrylic which attributes to the C– has already grafted on PNIPAm hydrogel. As the metal element does not have IR absorption, it shows no characteristic peaks of Ag NRs. 3.4. Differential scanning calorimetry DSC heating thermograms were used to investigate the effects of acrylic and Ag NRs content on the LCST of four samples (pure PNIPAm hydrogel and hydrogels with acrylic and different concentration of Ag NRs), results were recorded in Fig. 3b. It can be seen from the spectra that pure hydrogel (SI) showed LCST of 31.4 � C, while the peak positions of other samples all remain at 32.9 � C. This indicates that addition of acrylic may increase the LCST of hydrogel. It has been reported that the LCST depends on the ratio of hydrophilic (–CONH–) and hydrophobic (-CH(CH3)2) groups on the PNIPAm chains [34,35]. In this work, the adjunction of Ag NRs during the in-situ polymerization does not alter the ratio of both groups in the PNIPAm so that the resultant samples show the almost same LCST compared to each other. Furthermore, LCST of 32.9 � C is an appropriate temperature to distinguish between hot or cool outdoors, indicating that the hybrid hydrogel in this work is a suitable candidate for smart window devices on regulating indoor temperature.
Fig. 4. UV–visible–NIR transmittance spectra of four samples at 25 � C and 40 � C (The inset is the solar irradiance spectrum, filled area).
NRs hardly affected the light transmission at room temperature. When the temperature raised to 40 � C, all samples behaved transparent-toopaque transition and their transmittance has declined to varying de grees. The device with pure PNIPAm showed decreased transmittance of 48.58% in the visible region and its ΔTsol keeps at 26.88% (Table 2). Compared with sample I, PNIPAm hydrogel with acrylic has a much higher transmittance variation rate at 40 � C both in visible region and NIR region. When adding Ag NRs, it could be found that doping of Ag NRs makes a great deal on transmittance modulation and the concen tration of Ag NRs is a major factor affecting the transmittance. Ag NRs were initially attached to the surface of hydrogel. While the comparison of Fig. 2b and f showed a shrinkage deformation after thermal drying, the deformation of hydrogel caused the variation of Ag NRs. As tem perature rises above LCST, the hydrogel structure shrinks and deforms, leading Ag NRs to a stand-up variation (Scheme 2). This change enhances light scattering and increases solar modulation ability. Various Ag NRs concentrations lead to different light scattering abilities, resulting in variation on transmittance modulation capabilities. Comparing the three samples (II, III, IV) in Table 2, the transmittance modulation of each sample containing different concentrations of Ag NRs differed by about 10% in both the visible region and the NIR region. Further con firming Ag NRs could help promote transmittance modulation. Fig. 5a depicts solar light transmittance (200–2500 nm) profiles of PNIPAm-acrylic/Ag NRs hydrogel (sample IV) under various tempera tures. The two prominent peaks observed at 1450 and 1932 nm result from the hydrogen bond due to water trapped in hydrogel [36]. The absorption peak of water at 1450 nm is attributed to the O–H stretch in the water molecule, and the other peak at 1932 nm is due to a
3.5. Transmittance & modulation effect To investigate the thermal sensitivity properties, samples were sandwiched in two glass sheets as smart window devices. The trans mittance spectra of four samples at a wavelength of 200–2500 nm under 25 and 40 � C are reported in Fig. 4 and integral calculation results are summarized in Table 2. It can be clearly seen that sandwich-structure devices with pure PNIPAm showed a relatively high transmittance of 73.20% in the visible region compared with the other three samples under 25 � C. While the transmittance of the other three samples dis played around 62% at wavelength between 380 and 780 nm, and it also decreases significantly after phase transition. This may be attributed to the grafting of acrylic that influenced the refraction and absorption of light. Moreover, samples (II, III, IV) with different concentrations of Ag NRs showed close transmittance values, thus indicating the doping of Ag
Fig. 3. (a) FT-IR of four samples; (b) DSC of four samples. 5
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Table 2 Transmittance and transmittance modulation of four samples in the visible and NIR region. T ¼ 25 � C SI SII SIII SIV
T ¼ 40 � C
Tranasmitance modulation
Tlum
TNIR
Tsol
Tlum
TNIR
Tsol
ΔTlum
ΔTNIR
ΔTsol
73.20 62.57 62.07 61.36
76.83 67.49 67.39 65.85
74.75 64.69 64.36 63.29
48.58 20.04 12.99 3.43
46.82 28.79 15.30 4.85
47.87 23.83 13.99 4.05
24.62 42.53 49.08 57.93
30.01 38.70 52.09 61.00
26.88 40.86 50.37 59.24
combination of O–H stretch and H–O–H bending. When the temperature is lower than LCST, much more hydrogen bonds exist between water and PNIPAm polymer chains; as temperature goes up, the hydrogen bonds begin to break up and cause continuous phase separation. The oscillator strength of the O–H bond is proportional to the hydrogen bond energy; therefore, the intensity of the transmittance valley decreases in case where the existing hydrogen bonds become fewer [5]. As for PNIPAm-acrylic/Ag NRs, it maintained transparency below the LCST with the calculated Tlum of 61.36% at 25 � C and slightly reduced Tlum of 60.33% at 28 � C (Fig. 5b). While its Tlum dropped down to 41.55% when the temperature rose to 31 � C and further declined to 3.79% at 34 � C, suggesting the LCST occurred between 31 � C and 34 � C, in line with DSC measurement. Later the transmittance decreased slowly and both ΔTlum and ΔTsol reached plateau afterward with temperature increase. As a result, Tlum was significantly reduced from 61.36% to 3.43%, and ΔTsol was boosted to 59.24% (Fig. 5b). Such a dramatic transition is desired for promptly blocking a large amount of solar irradiance especially in
Scheme 2. Schematic of enhanced solar modulation based on a stand-up vari ation of Ag nanorods.
Fig. 5. (a) UV–visible–NIR transmittance spectra of SIV at different temperatures (The inset is the solar irradiance spectrum, filled area); (b) The luminous (Tlum), NIR (TNIR), and solar (Tsol) transmittances (solid lines) of SIV and corresponding transmittance modulations (ΔTlum, ΔTNIR, ΔTsol) (dashed lines) in response to the temperature.
Fig. 6. (a) Luminous transmittance (Tlum) at 25 � C and 40 � C, and solar transmittance modulation (ΔTsol) of SIV during 20 cycles; (b) images of smart window device that consists of SIV after 20 cycles at 25 � C and 40 � C. 6
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the visible and NIR region [37]. As in practical field applications, it is vitally important for smart windows to achieve a reversible optical transition. Hence, the reversible and repeatable optical switching properties of PNIPAm-acrylic/Ag NRs hydrogel (sample SIV) were investigated and results were shown in Fig. 6a. After 20 cycles, the Tlum at both high and low temperatures are consistent and ΔTsol remains unchanged. This indicates that both transmittance and solar modulation ability of such hybrid device should be reliable in an actual smart window application as long as good sealing to prevent water evaporation is achieved. Images of smart window de vice with SIV hydrogel were photoed at 25 � C and 40 � C after 20 cycles’ phase transition. The hydrogel exhibited excellent optical stability while maintaining clear light transmission.
reached 44 � C (16.9 � C increments), whereas device with SIV exhibited much lower increase rate about 0.61 � C/min and finally reached 36.3 � C (9.2 � C increments). This can be attributed to the solar energy shielding caused by phase transition of PNIPAm hydrogel and transmittance modulation influenced by Ag NRs structural changes. Unexpectedly, it appears a slight temperature decline after 6 min both in two hydrogel samples, which may indicate a net negative heat gain inside the tube induced by a sudden opacity change whereby the heat dissipation is more significant than the heat input from illumination. After 15 min illumination, temperatures of water under three devices all reached plateau and gradually dropped down to a similar value. Based on above experiments, PNIPAm-acrylic/Ag NRs hydrogel behaves an effective way on modulating solar energy and thus to be an excellent material for smart windows.
3.6. Infrared emissivity
3.8. Comparisons of Tlum and ΔTsol
To investigate the changes of infrared emissivity of hydrogel under different temperature conditions, infrared parameters of four samples were measured by IR-2 Dual-Band Infrared Emissivity Measuring In strument and the results were shown in Table 3. Four samples all showed high infrared emissivity above 0.9 both in the thermal infrared region (8–14 μm) and middle infrared region (1–22 μm). This may be attributed to the strong absorption of organic functional groups for infrared ther mal radiation. However, strong absorption capacity is equivalent to high heat dissipation, which is also effective in radiative cooling under thermal conditions. For pure PNIPAm hydrogel (SI), its infrared emis sivity is the highest among four samples, this is due to the rough surface caused by its porous network structure. Later when temperature goes up to 40 � C, its surface became smooth after phase transition and infrared emissivity value dropdown. Comparing the other three samples, hydrogels with acrylic exhibit smooth surface and relatively lower values. When the temperature exceeds the LCST, the emissivity value of each sample increases to a different extent, which is beneficial to ach ieve radiative heat dissipation at high temperatures. It can also be found that as the Ag NRs content increases, the value of infrared emissivity decreases and emissivity modulation becomes large. This indicates Ag NRs also have modulation ability on thermal management in the infrared region.
Fig. 8 displays a comparison of our work with recently reported smart window devices based on VO2 and hydrogel composites. As unilaterally pursuing superior solar modulation capability or ultrahigh luminous transmittance is meaningless, it is vitally important to consider high Tlum accompanied by high ΔTsol. For VO2 composites, both Tlum and ΔTsol all stayed at low level which is not suitable for applying in the smart windows. While after VO2 particles combining with ion liquid or hydrogel, the ΔTsol could reach about 30–35%. Interestingly, Lee and his group present a thermochromic ionogel which exhibits Tlum of 88% and ΔTsol of 57% [38]. However, these research all lack concerns of infrared emissivity in the thermal infrared region. In such a case, the current PNIPAm-acrylic/Ag NRs hydrogel with Tlum of 61.36% accompanied by ΔTsol of 59.24% and infrared modulation ability is extremely competi tive for the smart windows. 4. Conclusions In summary, a new type of full wavelength thermal management window device based on PNIPAm-acrylic/Ag NRs hydrogel has been investigated systematically in this paper. Through a comprehensive analysis of the hydrogel optical switching behavior, the mechanism for enhanced transmittance modulation ability and emissivity regulation ability was elucidated by studying the doping concentrations of different Ag NRs and grafting acrylic. With proper acrylic and Ag NRs concen tration, hydrogel can be fully transparent at room temperature with Tlum of 61.36%, and achieving high ΔTlum of 57.93% after phase transition. Meanwhile, the hybrid hydrogel showed ΔTNIR of 61.00% in the tem perature range of 25 � C–40 � C. The aspects of both luminous and NIR shielding characteristics would achieve dual regulation mode on ther mal management, which may be beneficial on application in smart windows for a better indoor comfort environment. Moreover, the solar modulation ability of hybrid hydrogel was found to be reversible and repeatable after 20 cycles of irradiation. Apart from solar modulation in the visible region and NIR region, infrared emissivity value of hybrid hydrogel got a slight increase from 0.947 to 0.958 (1–22 μm) when temperature rises above the LCST, which is effective for accomplishing radiative cooling indoors. While water-warming experiment also sup ports the point with 12.7 � C reductions of hybrid hydrogel compared with blank glass. Such large and full-wavelength solar modulation makes the hybrid hydrogel an ideal candidate for smart windows ap plications. And the new method for inorganic-organic assembly may give an inspiration for hybrid materials based on smart windows.
3.7. Energy saving performance In order to characterize the reduction of the heat flux from solar irradiance due to the phase transition of the hydrogel, a water-warming experiment was carried out to simulate indoor thermal management and visually demonstrate the energy-saving efficiency of the hybrid hydro gel. Three window devices were installed on chambers above three tubes with equal amount of water. The blank device with two quartz glasses was set as a control experiment on the left, sandwich-structure devices filling with SIII and SIV were set on the middle and right side, respec tively. Temperature changes were recorded in Fig. 7a under 1 sun, air mass 1.5 illuminations. At first, the initial temperatures of water under three window devices were all 27.1 � C. While the temperature of water under blank glass increased rapidly to 49 � C after light on for 15 min (21.9 � C increments). As for the other two sandwich-structure devices, device filling with SIII showed low increase rate about 1.13 � C/min and Table 3 Infrared emissivity of four samples in the thermal infrared region (8–14 μm) and middle infrared region (1–22 μm). 8–14 μm SI SII SIII SIV
1–22 μm
25 � C
40 � C
25 � C
40 � C
0.986 0.968 0.959 0.958
0.965 0.972 0.962 0.968
0.965 0.961 0.953 0.947
0.95 0.966 0.961 0.958
Author contributions section Gengyao Wei: Methodology, Supervision, Writing-Original Draft. Dongya Yang: Investigation, Resources. Tao Zhang: Funding acquisi tion, Supervision, Writing-review editing. Xuejie Yue: Data curation, Validation. Fengxian Qiu: Conceptualization. 7
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Fig. 7. (a) Temperature profiles of a thermometer inside the model chamber affixed with Blank glass (black), SIII (green) or SIV (blue) as the window; (b) compared thermal images from different irradiation time. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 8. Luminous transmittance (Tlum) and solar energy modulation (ΔTsol) of different smart windows: single-layered VO2 films [39-41], VO2/SiO2 com posite films [42-44], doped VO2 films [45–49], VO2/hydrogel films [5,25], VO2/IL-Ni-Cl films [50], and PNIPAm-acrylic/Ag NRs films in this work.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This work was financially supported by National Natural Science Foundation of China (21706100 and 21878132), Natural Science Foundation of Hebei Province (B2019108017), Xingtai Polytechnic College Application Innovation Project (20190106). Youth Talent Cultivation Program of Jiangsu University, High-Level Personnel Training Project of Jiangsu Province (BRA2016142) and Key Laboratory of Pollution Control and Resource Reuse Foundation (NO. PCRRF18003). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110336. 8
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