Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 12801−12808
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VO2@SiO2/Poly(N‑isopropylacrylamide) Hybrid Nanothermochromic Microgels for Smart Window Yu Wang,† Fang Zhao,*,† Jie Wang,† Ali Raza Khan,† Yulin Shi,‡ Zhang Chen,§ Kaiqiang Zhang,§ Li Li,† Yanfeng Gao,*,§,∥ and Xuhong Guo*,†,‡ †
State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China Engineering Research Center of Materials Chemical Engineering of Xinjiang Bingtuan, Key Laboratory of Materials Chemical Engineering of Xinjiang Uygur Autonomous Region, Shihezi University, Shihezi 832000, P. R. China § School of Materials Science and Engineering, Shanghai University, Shanghai 200444, P. R. China ∥ School of Materials Science and Energy Engineering, Foshan University, Foshan 528000, China
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‡
S Supporting Information *
ABSTRACT: VO2/PNIPAm hybrid thermochromic material has been verified as promising to improve optical performance of smart windows. However, VO2 can be easily oxidized especially when being dispersed in the hydrogel matrix, and also the nonuniform dispersion of VO2 in a host will worsen the material’s optical performance. Herein, pure VO2(M)@ SiO2 nanoparticles (NPs) were first prepared using a surfactant-free method. Then VO2(M)@SiO2/PNIPAm hybrid microgels were synthesized through suspension polymerization. The VO2(M)@SiO2 NPs in the hydrogel matrix exhibited obviously improved resistance to oxidation. Monodispersed hybrid microgel with uniformly distributed VO2(M)@SiO2 NPs (Sample I) was obtained. Meanwhile, compared with the best VO2 film reported, our composite microgel (Sample III) demonstrated a 164% increase in the solar modulating ability at a relatively high average luminous transmittance (38.4%). All these remarkable features of VO2(M)@ SiO2/PNIPAm microgel synthesized in our work will help a lot in the practical application of the inorganic−organic hybrid nanothermochromic materials. In the continuous pursuit of larger ΔTsol and Tlum, new materials or new structures are needed to maximize energy saving. Integrating VO2 with other organic materials to form a hybrid material has been proved to be a valid method. Zhou et al.10 integrated poly(N-isopropylacrylamide) (PNIPAm) hydrogel with VO2 nanoparticles (NPs), and successfully obtained a hybrid material with higher ΔTsol and Tlum (ΔTsol = 34.7%, Tlum,average = 62.6%). Similarly, Yang et al.11 dispersed VO2 NPs into a hydroxypropl-cellulose polymer matrix, and the obtained hybrid material showed good optical performance (ΔTsol = 36%, Tlum,average = 56%). However, VO2 is not a thermodynamically stable material, especially in the hydrogel. The homogeneous dispersion of VO2 in the hydrogel to form uniform hybrid material, is also required to improve both Tlum and ΔTsol.12 Bilayer coating is an effective method for enhancing thermal stability, monodispersity, and optical properties of par-
1. INTRODUCTION The global contribution of buildings to final energy consumption accounts for 20−40% and overtakes the energy consumption of transportation and industrial sectors. Because of the improvement of the indoor environment and enhancement of building services, more and more final energy is used in building sectors.1,2 Smart windows are designed to intelligently control the amount of heat and light transmitted through the glass when stimulated by electricity (electrochromism)3 or heat (thermochromism).4,5 Among these, thermochromic materials attract much attention because they can be used with zero-energy input and solar energy itself is utilized as a booster against solar heat.6 Vanadium dioxide (VO2) is one of the most attractive thermochromic materials.7 The best performance reported in the literature for smart window based on VO2 materials is 22.3% of the solar modulating ability (ΔTsol), with 42.8% of the average luminous transmission (Tlum,average).8,9 A tremendous amount of work has been focused on the optimization of film thickness, composition, doping, and multilayered structure design to improve its optical performance, which however still does not meet the demand of the practical applications yet. © 2018 American Chemical Society
Received: Revised: Accepted: Published: 12801
June 16, 2018 August 29, 2018 September 3, 2018 September 3, 2018 DOI: 10.1021/acs.iecr.8b02692 Ind. Eng. Chem. Res. 2018, 57, 12801−12808
Article
Industrial & Engineering Chemistry Research ticles,13−15 especially coating with SiO2. The inert barrier of SiO2 shell can prevent the VO2 NPs from being oxidized to V2O5 in humid environment,16 and the core−shell structure can also improve the monodispersity of nanoparticles due to alleviation of agglomeration. Gao et al.15 used a traditional Stober method to coat the magnetite nanoparticle Fe3O4 with a SiO2 shell. Due to the existence of the SiO2 shell, Fe3O4@ SiO2 NPs were almost monodispersed in the aqueous solution. The good dispersion in a host to form a uniform and transparent composite can improve the optical performance.8 Gao et al.16 modified the surface of VO2, which had good dispersion ability in the aqueous solution containing polyurethane. The modified particles resulted in comparable optical performance to high-quality VO2 films. Particularly, when the refractive index (RI) of the shell is lower than that of internal nanofillers (the RI of amorphous SiO2 is 1.5 vs 2.0 of VO2), the scattering effect caused by the RI mismatch between the polymer and the nanoparticles can be decreased, and the optical properties can be improved.17 In a word, coating with silica shell has three main advantages. First, it can protect the particles from oxidation and improve the mechanical stability of the nanoparticles. Second, the SiO2 shell has the capability to improve the monodispersity of particles in the bulk phase (e.g., aqueous solution and polymer matrix). Finally, the lower RI of SiO2 shell can enhance the optical performance of the resulted material, especially the luminous transmittance. In this paper, a novel process for distributing the primary VO2(M)@SiO2 NPs into the monodispersed PNIPAm microgels is proposed. The preparation procedure is described as follows. First, highly pure VO 2 (M)@SiO 2 NPs were synthesized through a method previously reported by our group with few byproducts.18 After that, VO2(M)@SiO2 NPs were preprocessed in a poly(vinylpyrolidone) (PVP) aqueous solution to adsorb PVP. Then, VO2(M)@SiO2/PNIPAm microgels were prepared via suspension polymerization. A class of VO2(M)@SiO2/PNIPAm hybrid microgels having different nanostructures were synthesized with different addition amounts of VO2(M)@SiO2 NPs and different concentrations of PVP solution. The improvement of inoxidizability of VO2(M) was validated, and the structure, morphology, optical properties, and phase transition speed of these hybrids were also investigated.
the surface of VO2(M) NPs with ammonia−water as the catalyst, resulting in a well-defined VO2(M)@SiO2 core−shell structure. 2.3. Synthesis of VO2(M)@SiO2/PNIPAm Microgel Composite Films. Figure 1 is a schematic illustration of the
Figure 1. Schematic illustration of the preparation of VO2@SiO2/ PNIPAm hybrids. The top left dashed box shows the core−shell structure of VO2(M)@SiO2 NPs. (The green coating represents SiO2 shell and the blue core represents VO2.)
route to prepare VO2(M)@SiO2/PNIPAm hybrid materials. A known amount of VO2(M)@SiO2 NPs were added into a 50 mL aqueous solution with 0.1−0.2 g PVP. After 24 h of magnetic stirring, the mixture was transferred to a three-necked flask. 2.5 g of NIPA and 0.025 g of BIS were added into the flask under stirring conditions (300 rpm) at 35 °C followed by continuous stirring for 30 min. After a transparent and clear solution was obtained, the solution was purged with N2 and heated to 60 °C. Then 2.5 mL of KPS solution with a concentration of 0.01 g/mL was added, and suspension polymerization was carried out at 60 °C for 12 h. The resultant hybrids were collected into a glass bottle for characterization. Because there was no postprocessing step, the final concentration of the polymer solution was around 2.5 wt %. Samples prepared with different PVP contents (2, 3, and 4 g/L) and VO2(M) NP contents were successfully made. 2.4. Characterization of Hybrids. 2.4.1. TEM and ICP Tests. The morphology of VO2(M)@SiO2 NPs and VO2(M)@ SiO2/PNIPAm hybrids were observed using a transmission electron microscope (TEM, JEOL2100F, JEOL, Japan) operating at 200 kV with an energy-dispersive spectrometer attachment (EDS). The content of vanadium in the hybrids was determined by inductively coupled plasma (ICP, Thermoelectric Corporation, IRIS Intrepid, America). The actual contents of VO2(M) NPs in the hybrids were then calculated from the ICP results. 2.4.2. Transmittance Spectrum Tests. The transmittance spectra at normal incidence irradiation from 350 to 2600 nm were monitored in a glass cuvette using a UV−vis−near-IR spectrometer (U-4100, Hitachi, Japan). A detailed description of characterization of optical properties was provided in our previous work.18,19 2.4.3. Response Time Tests. The response performance of the prepared samples were monitored by the UV−vis−near-IR spectrometer with a water bath to control system temperature. Temperature and transmittance at 1100 nm as a function of
2. EXPERIMENTAL SECTION 2.1. Materials. VO2(M) NPs was synthesized as described in our previous work.8 Ammonium water, ethanol, and isopropanol were purchased from Sinopharm Chemical Reagent Co., Ltd. Tetraethyl orthosilicate (TEOS, 99%), Nisopropylacrylamide (NIPA, 98%), and N,N′-methylenebis (acrylamide) (BIS, 99%) were obtained from Adamas Reagent Co., Ltd. PVP (K30, Mw = 58 000 g/mol) and potassium persulfate (KPS, 99%) were purchased from Aladdin Reagent (China) Co., Ltd. All the reagents were used without further purification. 2.2. Synthesis of VO2(M)@SiO2 NPs. VO2(M) NPs was modified with SiO2 shell according to our previous work.13 Typically, 0.2 g VO2(M) NPs were ground with ethanol to increase the amount of hydroxyl groups on the NPs. Then these VO2(M) NPs were dispersed in water to ensure that the NP exterior was surrounded by water. Afterward, these pretreated VO2(M) NPs were added into 500 mL isopropanol followed by the addition of 3 mL ammonia−water. Then, TEOS solution was added dropwise and TEOS hydrolyzed on 12802
DOI: 10.1021/acs.iecr.8b02692 Ind. Eng. Chem. Res. 2018, 57, 12801−12808
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Industrial & Engineering Chemistry Research
(Figure 2d) also showed that the VO2(M) surface was modified, which is indicated by the migration of wavenumber and new wave numbers. The broad vibrational band around 3400 cm−1 is the peak of the functional groups O−H and Si− OH which is related to the surface of SiO2.21 The absorption peak of VO bonds is blue-shifted, further verifying that VO2(M) was coated with SiO2. In a word, the results of TEM, EDS, and FT-IR validate the VO2(M)@SiO2 core−shell structure. Figure 3a, b shows the oxidation curves for VO2(M) and VO2(M)@SiO2 respectively, which both indicate a strong
time during the switching process were recorded. Both the cooling and heating tests were conducted the same condition. 2.4.4. FT-IR Tests. Fourier transform infrared (FT-IR) spectra of VO2(M) and VO2(M)@SiO2 were monitored on an FT-IR spectrometer (IR-430, JASCO, Japan). 2.4.5. DSC Oxidation Tests. The oxidation experiments of VO2(M)@SiO2 nanoparticles were carried out by differential scanning calorimetry (DSC200F3, NETZSCH, Germany). Samples were placed in an open aluminum crucible, and the purge gas formed the oxidation atmosphere. The measurements were performed by heating the temperature up to 450 °C with different heating rates of 5, 10, 15, and 20 °C/min, respectively. According to the literature,20 enthalpy is a characteristic value for the property of a material, and the peak area of the oxidation curve is equal to the heat enthalpy. Thus, the oxidation percentage α, which is defined as the proportion of the oxidized phase (e.g., V2O5) in the whole nanoparticle, can be calculated with the following equation: T
( ddHt )dt α= T ∫T ( ddHt )dt ∫T
1
2
1
× 100% =
ΔHT × 100% ΔH
(1)
where T1 and T2 denote the temperatures at which crystallization begins and ends, respectively, and T shows the temperature value which is used to find the oxidation ratio of the samples. ΔHT is the enthalpy between T1 and T and ΔH represents the enthalpy between T1 and T2.
3. RESULTS AND DISCUSSION 3.1. Enhancement of Antioxidation Properties with SiO2 Shell. Herein, we used a previously reported surfactantfree method to synthesize pure VO2(M)@SiO2 nanoparticles.14 This method favored the formation of uniform and smooth VO2(M)@SiO2 structure which accounted for over 90% with few byproducts. Figure 2a−c shows the HRTEM images of the silica-coated VO2 NPs. The dark contrast in the central part showed the fine-quality crystal VO2(M) (column-like) with a size between 5 to 100 nm. It can be seen in Figure 2b, c that the VO2(M) crystal was coated by a smooth SiO2 shell around 2 nm thick. The FT-IR results
Figure 3. Results for antioxidation tests. DSC curves of (a) VO2(M) and (b) VO2(M)@SiO2 samples with different heating rates. (c) Oxidation conversion curves (oxidation percentage vs temperature) of VO2(M) and VO2(M)@SiO2.
exothermic reaction between 300 and 350 °C. The heat flux peaks for both samples shifted to higher temperature as the heating rate increased, which means that higher temperature was needed to overcome the larger energy barrier.22 With eq 1, the oxidation percentages of VO2(M) and VO2(M)@SiO2 as a
Figure 2. Characterization of the synthesized VO 2(M)@SiO2 nanoparticles. Parts (a−c) are HRTEM images and the right-bottom image in (c) is the EDS result for the sample. (d) FT-IR spectra of VO2(M) and VO2(M)@SiO2. 12803
DOI: 10.1021/acs.iecr.8b02692 Ind. Eng. Chem. Res. 2018, 57, 12801−12808
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Industrial & Engineering Chemistry Research function of temperature at a heating rate 10 °C·min−1 were calculated, respectively. As shown in Figure 3c, the distance between the VO2(M) oxidation conversion curve and that of VO2(M)@SiO2 was around 20 °C. This result indicates the synthesized VO2(M)@SiO2 NPs had improved the antioxidation properties. Moreover, when the VO2(M)@SiO2 NPs are mixed with hydrogel, the SiO2 shell will protect the mechanical stability of VO2(M) NPs in this hyper-humidity environment which is beneficial for commercialization of smart window system. 3.2. Preparation and Characterization of Hybrids. Description of the different organic−inorganic hybrid samples is shown in Table 1. Due to the relatively large VO2(M) NP Table 1. Description of the Organic−Inorganic Hybrid Nanothermochromic Materials (Samples I−IV) Investigated in This Work sample
CPVPa (g/L)
CVO2(M)@SiO2b (mg/L)
CVc (mg/L)
CVO2(M)d (mg/L)
I II III IV
2 3 3 4
250 250 500 500
38.94 49.52 65.10 70.26
63.40 80.64 105.99 114.39
Figure 4. TEM and high resolution TEM (HRTEM) characterizations of sample I. Parts (a−c) are TEM images of thermosensitive sample I, and (d) is HRTEM image for sample I. The SiO2 shell and the interplanar distance are marked in (c) and (d), respectively.
oxidized during the synthetic process. Herein, we obtained monodispersed microgels with several well-dispersed VO2(M) @SiO2 cores in each microgel. As discussed above, the loading of VO2(M)@SiO2 NPs in PNIPAm hydrogel was still too low. To address this issue, samples II, III, and IV were synthesized with increased PVP contents (Table 1). Figure 5 presents the TEM images for
a
The mass content of PVP used to stabilize VO2(M) NPs. bThe mass content of VO2(M)@SiO2 NPs used to prepare the hybrids. cThe actual V content in the hybrids determined by ICP. dThe actual content of VO2(M) calculated from CV.
diameter range from 5 to 100 nm, the NPs are not so uniform as to be incorporated with PNIPAm microgel, and the nonuniform NP size distribution will lead to agglomeration and sedimentation. To address this issue, preliminary investigations were conducted (see Section S1.2 of the Supporting Information, SI) and PVP was opted to modify the surface of VO2(M)@SiO2 NPs, which is also a common method for the modification of VO2(M) NPs.17 Due to the SiO2 shell, the surface of VO2(M)@SiO2 NPs possessed a large number of hydroxy groups. Consequently, PVP was able to be absorbed onto the VO2(M)@SiO2 NP surface via interaction between carbonyl groups on PVP and hydroxy groups on NPs. In addition, PVP acted as the stabilizer and surfactant to synthesize the VO2(M)@SiO2/PNIPAm hybrid nanothermochromic material. Hybrid microgel sample I was first synthesized with a PVP concentration of 2 g/L. A detailed analysis for the morphology of sample I is shown in Figure 4. The TEM images show that about 8−10 VO2(M)@SiO2 NPs were well-dispersed in PNIPAm microgel without significant aggregation, and the PNIPAm microgels had a quasi-spherical morphology with a narrow size distribution. Large VO2(M)@SiO2 NPs with a size above 20 nm were hard to be incorporated by PNIPAm microgel, and we observed that these large VO2(M)@SiO2 NPs ultimately subsided during the polymerization process.23,24 Meanwhile through TEM characterization we observed that only small NPs were included in the PNIPAm microgel. As shown in Table 1, for sample I it was calculated that only 32.2% of all the used VO2(M)@SiO2 NPs were dispersed into the polymer matrix. The mean particle size of VO2(M)@SiO2 NPs in the polymer matrix was around 10 nm with a 0.59 nm thick SiO2 shell (Figure 4c). Figure 4d shows a clear and ordered lattice fringe corresponding to the (120) crystalline plane of VO2. It indicates that the VO2 in the VO2(M)@SiO2 NPs were not
Figure 5. TEM micrographs of the products prepared from varying amounts of VO2(M)@SiO2 NPs in PVP solutions (CPVP = 3 g/L): (a) and (b) are sample II (CVO2(M) = 80.64 mg/L); (c) and (d) are sample III (CVO2(M) = 105.99 mg/L).
samples II and III, which were prepared with the same PVP concentration 3 g/L but with two different VO2(M)@SiO2 NP concentrations. For sample II (Figure 5a, b), VO2(M)@SiO2 NPs were not homogeneously dispersed in the microgel. Meanwhile, agglomeration of NPs and empty PNIPAm microgels were also observed. For sample III (Figure 5c, d) with a higher VO2(M)@SiO2 NP content, more VO2(M)@ SiO2 NPs were observed to be incorporated into the PNIPAm microgel. And most of the VO2(M)@SiO2 NPs were located on the surface of the PNIPAm microgel. When the concentration of PVP was increased further to 4 g/L (sample 12804
DOI: 10.1021/acs.iecr.8b02692 Ind. Eng. Chem. Res. 2018, 57, 12801−12808
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Industrial & Engineering Chemistry Research Table 2. Thermochromic Properties of PNIPAm Microgel, Sample I, II, III, and IV TIR (%)
Tsol(%)
samples
20 °C
40 °C
60 °C
80 °C
20 °C
40 °C
60 °C
80 °C
PNIPAm Sample I Sample II Sample III
30.6 29.9 30.1 31.2
1.2 2.4 6.8 13.2
1.1 3.6 8.3 13.8
1.2 3.2 6.7 8.7
67.1 69.0 62.3 72.4
2.3 3.3 7.7 14.9
2.0 4.5 9.3 15.7
2.1 4.1 7.5 9.7
IV), the loading capacity of VO2(M)@SiO2 NPs was improved (Table 1). The higher concentration of PVP could destroy the morphology of the PNIPAm microgel (Figure S5 in the SI) and thus influence the optical performance of the material. Therefore, 4 g/L of PVP content is not suitable for synthesizing the hybrids in our work. Herein, we obtained monodispersed microgels with VO2(M)@SiO2 NPs attached to the microgel surface and improved the loading capacity of the NPs to some extent. 3.3. Energy Saving Performance of Hybrids. Table 2 and Figure 6 show the thermochromic properties of PNIPAm microgel and the synthesized nanothermochromic hybrids (Samples I and II) in the wavelength range between 350 and 2500 nm. It can be seen that PNIPAm had a sharp and total phase transition behavior, especially when it was dispersed into the low-viscosity water solution. In the meantime, a low luminous transmission after phase transition was observed, due to the thick measuring cell in our experiment (3 mm). Some previous research reported that the optimum thickness for PNIPAm was 52 μm, in favor of visible light transmission with good solar modulation properties.25 It can be expected that if the thickness of our hybrid material is proper, then the hybrid material is applicable for smart windows. However, the thickness parameter will not be discussed in this paper, and we will focus on the interaction between VO2 NPs and PNIPAm, for example the change of optical performance due to the addition of pure VO2 NPs in PNIPAm microgel. Figure 6a, b illustrates the increase in transmittance at 1250 nm from 0% to 25% at 40 °C by the addition of VO2(M)@SiO2 NPs. Moreover, a higher concentration of VO2(M)@SiO2 NPs in the hybrids (sample III) leaded to higher transmittance of 60% at 1250 nm under the temperature of 40 °C (Figure 7). This trend confirms that we have a well-defined composite structure having a negative effect on the optical performance to some extent. The more VO2(M)@SiO2 NPs being dispersed in PNIPAm microgel there are, the stronger the effect of NPs will be on the swelling−deswelling process of the microgel, which directly influences the optical performance. As compared to PNIPAm microgel, in the IR range, it is of interest to observe that the transmittance lines of hybrid samples showed a different changing trend with respect to temperature (Figure 6). To elucidate this intriguing phenomenon more straightforwardly, IR transmittance (TIR) values were calculated. As shown in Table 2, the TIR value dramatically reduced from 29.9% to 2.4% for sample I and from 30.1% to 6.8% for sample II, respectively, when the temperature was increased from 20 °C to 40 °C. But when the temperature was increased from 40 °C to 60 °C, the TIR value underwent a strange increase to 3.6% for sample I and to 8.3% for sample II, respectively. At a further elevated temperature of 80 °C after the phase transition of VO2(M), the IR transmittance of the hybrids fell to 3.1% for sample I and 6.7% for sample II.
Figure 6. Temperature dependence of the optical transmittance spectra of the three samples: (a) PNIPAm, (b) sample I, and (c) sample II between 20 °C and 80 °C.
For the transmittance curves of sample III, the abovedescribed interesting trend also existed. The possible cause is due to the variable structure of bybrid microgel during the 12805
DOI: 10.1021/acs.iecr.8b02692 Ind. Eng. Chem. Res. 2018, 57, 12801−12808
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Industrial & Engineering Chemistry Research
38.4% (best reported performance of VO2 film is 23.7% at Tlum = 32.4%8) is promising. 3.4. Response Speed and Cyclic Stability. Phase transition speed is also a critical parameter for a thermochromic material in the smart window system. Here we chose microgel instead of conventional hydrogel for the preparation of hybrid thermochromic material as microgel was reported to exhibit a rapid phase transition.26 The detailed description of the method to measure thermosensitive response performance was presented in our previous work.18 As shown in Figure 9a,
Figure 7. UV−vis−near-IR spectra of pure VO2(M) (dashed lines) and sample III (solid lines) at different temperatures. The area with sparse slashes is the solar modulation performance of the sample III.
swelling−deswelling process. After phase transition of PNIPAm microgel (40 °C), some VO2(M)@SiO2 NPs will converge on the microgel surface (as shown in Figure 5b). These VO2(M) NPs shows a high infrared transmittance (black dashed line in Figure 7) before phase transition, which however is lowered after phase transition of VO2(M) (red dashed line in Figure 7). Therefore, these VO2(M)@SiO2 NPs on the microgel surface will increase the infrared transmittance of hybrids at 60 °C (Figure 7), and a significant decrease of infrared transmittance will occur above 68 °C. The solar regulation efficiency (ΔTsol) of PNIPAm microgel basically comes from a giant distinction in the visible region, whereas that of VO2(M)@SiO2 NPs is mainly based on a huge difference in the infrared range. For example, sample III, in the IR range, had an ΔTIR (IR modulating ability) value of 5.1% which was ascribed to VO2(M)@SiO2 NPs (yellow area in Figure 8), and in the visible region sample III had an ΔTIR
Figure 9. (a) Temperature and transmittance changes as a function of time through cooling or heating across lower critical solution temperature (LCST) for sample III. The inset shows the first-order derivatives of transmittance versus time in cooling and heating process. (b) Durability test of sample III between 20 °C and 80 °C.
the duration time for transparent−opaque transition period of sample III was around 150 s, which is further confirmed by the dTr/dt plot. Figure 9b illustrates the durability test of the obtained thermosensitive hybrid (sample III), showing that in the six cycles there existed only slight fluctuations in the values of ΔTsol and Tlum. The reason for these fluctuations is primarily the mishandling of time for the swelling−deswelling process (human error). Besides this, after durability test, all the samples were kept transparent and homogeneous, without any precipitation phenomenon (see SI Figure S3). The sample III synthesized here exhibited excellent reversible phase transition behavior, making it reliable in the practical application.
Figure 8. Changes of Tlum, ΔTsol, ΔTIR, and ΔTlum at different temperatures for sample III.
value of 74.6% which was due to the phase transition behavior of PNIPAm and the light absorption of water. As a result, the hybrids synthesized in our work have enhanced modulation ability in both the visible and infrared ranges, remarkably increasing ΔTsol to 62.7%. The performance of the hybrid sample III with a 164% increase in ΔTsol and Tlum, average up to 12806
DOI: 10.1021/acs.iecr.8b02692 Ind. Eng. Chem. Res. 2018, 57, 12801−12808
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Industrial & Engineering Chemistry Research
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4. CONCLUSIONS In this work, for the first time, VO2(M)@SiO2/PNIPAm hybrid microgels were synthesized. The antioxidation property of VO2(M) was improved through coating with SiO2, which can retain the good optical performance of VO2(M) even in humid environment. By changing the experimental parameters, such as the surfactant PVP content, hybrid samples with different VO2(M)@SiO2 loading capacities were prepared through suspension polymerization. Well dispersed hybrid structure was obtained with a suitable PVP content of 2 g/L (sample I). Meanwhile, hybrids (sample III) with a higher loading of VO2(M)@SiO2 NPs were synthesized by employing a higher PVP concentration 3 g/L, which embodied excellent optical performance with improved solar modulation property (ΔTsol = 62.7%) and an increased ΔTIR of 5.11%. In the meantime, we chose the PNIPAm microgel instead of the conventional hydrogel resulting in a fast phase transition speed and superior durability and reversibility properties. In conclusion, this new hybrid VO2(M)@SiO2/PNIPAm thermochromic material is a promising choice, especially in the smart window system.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b02692.
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The X-ray diffraction pattern of VO2(M) and VO2(M)@ SiO2; effect of surfactant type on VO2(M) dispersion; the transparent−opaque transition of the smart window; the phase transition behavior of VO2 and VO2@SiO2; the optical performance of Sample I, II, and III and pure PNIPAm microgel; TEM image of hybrids with 4 g/L PVP content; and comparison of optical performance of previously reported results and my hybrid sample (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (F.Z.). *E-mail:
[email protected] (Y.G.). *E-mail:
[email protected] (X.G.). ORCID
Fang Zhao: 0000-0003-0570-7729 Li Li: 0000-0001-5100-734X Yanfeng Gao: 0000-0001-7751-1974 Xuhong Guo: 0000-0002-1792-8564 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully thank the financial support from the National Natural Science Foundation of China (21476143, 51773061, and 5171101370), 111 Project Grant (B08021).
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DOI: 10.1021/acs.iecr.8b02692 Ind. Eng. Chem. Res. 2018, 57, 12801−12808
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DOI: 10.1021/acs.iecr.8b02692 Ind. Eng. Chem. Res. 2018, 57, 12801−12808
