PVP Nanowires and Superior Near-Infrared Filtration

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Applications of Polymer, Composite, and Coating Materials

Electrospun CuS/PVP Nanowires and Superior Near-Infrared Filtration Efficiency for Thermal Shielding Applications Young-Tae Kwon, Seung Han Ryu, Ji Won Shin, Woon-Hong Yeo, and Yong-Ho Choa ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22086 • Publication Date (Web): 21 Jan 2019 Downloaded from http://pubs.acs.org on January 23, 2019

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ACS Applied Materials & Interfaces

Electrospun CuS/PVP Nanowires and Superior Near-Infrared Filtration Efficiency for Thermal Shielding Applications Young-Tae Kwon†‡, Seung Han Ryu‡, Ji Won Shin‡, Woon-Hong Yeo†§*, and Yong-Ho Choa‡* † George W. Woodruff School of Mechanical Engineering, Institute for Electronics and Nanotechnology, Georgia Institute of Technology, Atlanta, GA 30332, USA. ‡ Department of Materials Science and Chemical Engineering, Hanyang University, Ansan 15588, South Korea. § Center for Flexible and Wearable Electronics Advanced Research, Wallace H. Coulter Department of Biomedical Engineering, Parker H. Petit Institute for Bioengineering and Biosciences, Institute for Materials, Georgia Institute of Technology, Atlanta, GA 30332, USA. *Address correspondence to [email protected] and [email protected]

Abstract Selective filtration of near-infrared (NIR) region is of primary importance to energy saving via thermal shielding. However, uniform coating of highly effective nanomaterials on flexible substrates remains very challenging. Here, we introduce new materials processing and fabrication methodologies that manufacture electrospun copper sulfide/polyvinylpyrrolidone (CuS/PVP) nanowires for enhanced thermal shielding efficiency. Electrospinning offers well-dispersed CuS 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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nanoparticles in a thermal shielding film, which is not achievable in typical solution coating processes. Directly deposited CuS/PVP nanowires on a flexible polymer membrane are enabled by a fluorination treatment that decreases interfacial electrostatic repulsion. Monitoring of in-situ temperature change of a box shielded, CuS/PVP nanowire film demonstrates excellent NIR shielding efficiency (87.15%). Direct integration of the film with a model car and exposure to direct sunlight demonstrates about twice-higher shielding efficiency than commercial tungstenoxide films. Overall, the comprehensive study of nanomaterial preparation, surface treatment, and integration techniques allows to fabricate highly flexible and reliable thermal shielding films.

KEYWORDS: Electrospinning, CuS/PVP nanowires, Near-infrared filtration, Thermal shielding, and Flexible film

Introduction Over the last century, explosive population growth and industrialization has resulted in depletion of fossil fuels and serious environmental pollution. Therefore, energy saving and efficiency are of vital importance for buildings and automobiles in both industrial and transportation sectors.1-3 More than 50% of the total energy is spent on air conditioning and heating, and a significant amount of this energy is lost due to poor thermal shielding and/or insulation of windows.4 To minimize energy loss, window films integrated with thermal shielding materials have been intensively investigated to filter near-infrared (NIR) in the solar spectrum. Considering the use in buildings and automobiles, thermal shielding materials require high transparency and superior block in NIR because ~52% of thermal energy is distributed from the NIR region.5 Recent research has studies various materials for filtering the NIR radiation, including metals (Ag and Au)6-7 and

2 ACS Paragon Plus Environment

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metal-doped semiconductors (BiPO3, VO2, tantalum octahedral clusters, and WO3)8-15. These studies have used either expensive metals or complicated doping procedures to design other materials. Here, an abundant and nontoxic material, copper sulfide (CuS), has been discovered that it exhibits high hole concentration (1021 cm-3) to yield strong absorption of localized surface plasmon resonance (LSPR) in the NIR region.5, 16 A common manufacturing method of thermal shielding materials for window applications in buildings and automobiles includes dispersing the particles into a polymeric solvent and solution coating on substrates.11, 14, 17-18 The biggest challenge of this process is on dispersibility of CuS because aggregated nanoparticles decrease LSPR.19 As an alternative method, electrospinning that utilizes electrostatic interactions showed a possibility to deposit polymer, inorganic, and inorganicorganic composite nanowires (NWs).20 The main advantage of the electrospinning technique is on the high dispersion capability of the composite NWs that could contain various nanoparticles (NPs) due to rapid evaporation of solvent ejected from a nozzle tip.21-24 However, a remaining challenge is to control the electrostatic repulsion force that limits direct deposition of electrospun NWs on a non-conductive polymer substrate.25-28 Here, we present newly developed surface coating methodologies and functional nanomaterials for direct deposition of a hybrid structure, inorganic-organic CuS/polyvinylpyrrolidone (PVP) on flexible polymers of polyethylene terephthalate (PET) and polyethylene naphthalate (PEN). A suface treatment technique using fluorination reduces electrostatic repulsion, which enables successful electrospinning of CuS/PVP NWs to form a thermal shielding film. This structure shows high optical transparency in visual wavelength (Vis) and excellent bandgap and LSPR absorption in both ultraviolet (UV) and NIR regions. In addition, the fabricated film on a polymer membrane is flexible, while maintaining optical efficiencies. We demonstrate enhanced thermal



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shielding efficiency of the film via comparison of in-situ temperature change with a commercial cesium-doped tungsten oxide and quartz glass.

Results and Discussion Figure 1 captures the overview of the newly developed materials processing strategy to fabricate a thermal shielding film. Electrospinning technology, explored in this work, allows direct integration of CuS NPs with PVP NWs, which makes a hybrid-structured film on a fluorinated flexible surface. The crystallinity and optical properties of the prepared Cu NPs appear in the Experimental Section.5 The high-resolution transmittance electron microscopy (HRTEM) image in Figure 1a shows that the lattice spacing of two adjacent crystal planes is 0.30 nm, corresponding to the (102) plane. This plane direction is matched with the main peak (JCPDS reference: 06-0463, Figure S1a,) of X-ray diffractometer (XRD) result; the average size of CuS NPs is 9.67 ± 0.68 nm. The optical spectra, ranging from 200 to 1400 nm, reveal that the aqueous CuS NPs solution for electrospinning has strong band gap and LSPR absorbance in UV and NIR regions (Figure S1b). To fabricate a thermal shielding film, we utilized an electrospinning technique. Generally, the electrospinning technique is not suitable for material deposition on an insulating surface due to the interfacial electrostatic repulsion.20 Thus, many prior studies added a transfer process after the metal deposition.26-28 This issue was resolved by employing a fluorination surface treatment, which is a simple and scalable approach to provide an antistatic property (Figure 1b). The surface charge of the fluorination-treated substrate becomes relatively negative, while the electrospun CuS/PVP NWs have positive charges, which results in highly reliable, successful deposition of NWs. A series of HRTEM images in Figures 1c-e captures CuS NPs that are uniformly embedded in the PVP NWs without aggregation. To avoid the high surface energy-enabled aggregation of NPs29,


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we utilized the PVP matrix in conjunction with electrospinning. The organic PVP played the key role in lowering surface energy of NPs19, 30-31 of the hybrid structure, which resulted in a nicely dispersed CuS NPs in PVP NWs through electrospinning. Figure 2 summarizes optical properties of the deposited CuS/PVP NWs on glass substrates. For optimization of the film fabrication, we varies electrospinning times (30, 60, 90, 120, 150, 180, and 210 minutes), which results in different densities of the CuS/PVP NWs (Figure 2a). Increased time of electrospinning makes a darker surface due to the densely packed NWs. Figure 2b presents transmittance spectra of fabricated films from UV to NIR regions; wavelengths of UV, Vis and NIR are 300 ~ 400 nm, 400 ~ 800 nm, and 800 ~ 2400 nm, respectively. Even though a long spinning time decreases the overall transmittance, the optical tendency is distinct in Vis region, along with the filtration in UV/NIR. Regardless of the spinning time, the structural morphology of electrospun NWs is intact (Figure S2) with the average diameter of 50 ~ 60 nm (Figure S3). In addition, we measured the optical transmittance at wavelength 550 nm and haze factor (Figure 2c and Table S1). The optical haze factor, ratio of diffusely transmitted light to the whole transmitted light (haze = Tdiffuse/Ttotal),32 is a crucial parameter for the diverse optical film of windows. The haziness increases from 0.05 to 26.73% with increased spinning time. Thermal shielding films for building or automobile windows require a low haze factor (typically 2 ~ 4%) to ensure comfort for the human eye and vivid visibility.33-34 A plot based on Beer-Lambert exhibits a linear correlation, indicating that the film transmittance depends on spinning time (Figure S4). Collectively, the materials characterization and deposition method demonstrates a highly reliable fabrication of thermal shielding films with the optimal processing time (90 minutes), which yields high transmittance (68.8% at 550 nm), low haze factor (1.89%), and film thickness of 13 μm (Figure S5).



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Figure 3 collects data of Fourier transform infrared (FT-IR) transmittance and structural configuration of electrospun CuS/PVP NWs on different substrates, including PEN and PET. Figure 3a indicates broad peaks at 1085, 1250, and 1340 cm-1, related to the stretching vibration of carbon–fluorine bond (C-F), and a peak at 1710 cm-1 for carbon-oxygen bond (C-O) stretching.35 The transmittance data clearly shows that the interfacial surface has a successfully functionalized fluorination, resulting in the negatively charged flexible polymer surface. The field emission scanning electron microscopy images (FE-SEM) in Figure 3b-d captures the interconnected, electrospun NWs, deposited on fluorinated glass, PET and PEN substrates. Figure 4 presents optical properties of thermal shielding films, measured by a UV-Vis-NIR spectrometer. The electrospun CuS/PVP NWs films on glass (Figure 4a), PET (Figure 4b), and PEN (Figure 4c) are highly transparent and the background can be clearly seen through the fabricated heat filtration films. Among them, examples on the flexible substrates of PET and PEN show the electrospinning capability for fabrication of mechanically compliant films. Figure 4d compares the optical spectra of CuS/PVP NWs on three substrates with only CuS NPs on PET and the commercial Cs-doped WO3 (CWO) on glass. The concentration of CuS NPs loaded on the film is same as 1%. To judge the performance of transparent heat insulation, we calculated the efficiencies in UV, Vis, and NIR regions as follows:36 400

𝑆𝑈𝑉 = 100% ―

∫300𝑇(𝜆)𝑑𝜆 400 ― 300

𝑇𝑉𝑖𝑠 = %𝑇 𝑎𝑡 550 𝑛𝑚

(1) (2)

2400

𝑆𝑁𝐼𝑅 = 100% ―

∫800 𝑇(𝜆)𝑑𝜆 2400 ― 800

(3)

where SUV and SNIR are the average shielding rate in UV and NIR regions, TVis is the transmittance


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in Vis region, and T(λ) is a function curve with wavelength as independent variable. Table 1 summarizes SNIR, Haze, SUV, and TVis values from 5 types of samples, including CuS/PVP NWs on glass, on PEN, on PET, CuS NPs on PET, and CWO on glass. Additional information about the interaction mechanism of light and the thermal shielding materials in NIR region is shown in Supporting Note S1, Figure S6 and Table S2. Although there are slight variation of values according to substrates, three thermal shielding films composed of CuS/PVP NWs show superior properties compared to easily aggregated CuS NPs on PET (Figure S7). In addition, the flexible film with CuS/PVP NWs on PET has consistent performance upon cyclic bending up to 10,000 times (Figure 4e). The measured NIR shielding efficiency and haze factor show negligible change with the bending test (180-degree bending with the radius of curvature 15 mm; details in Figure S9). Even with 10,000 cyclic bending, no mechanical fracture was observed on the thermal shielding film. For a potential application aiming at windows, we conducted a reliability test of a CuS/PVP NW film (PET substrate) by evaluating transmittance and haze at high temperature (85oC) and high humidity (85%). During the 1-week long test, no significant change was observed in the NIR shielding efficiency and haze factor (Figure 4f). Figure 5 summarizes the results of high-fidelity in-situ temperature monitoring on thermal shielding films, attached in a model car and building window. For validation of the shielding efficiency, we mounted the film of CuS/PVP NWs on PET onto the window of a model car (1:14 scale) that compares thermal shielding performance with no film and commercial CWO film (Figures 5a-c). Initially, the temperature in the car showed ~34 oC. After 1 hour of exposure under the direct sunlight, the measured temperature was increased up to 43.9, 39.8, and 37.6 oC for the car without any film (Figure 5a), with a commercial film (Figure 5b), and with the CuS/PVP NWbased film (Figure 5c). The result indicates that thermal shielding films are helpful to block the



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sunlight; specifically, the newly developed NW film has about twice-higher efficiency than the commercial film. In addition, we conducted another set of experiments that mimic thermal shielding efficiency in a building. A simulated dark box was designed to embed three films (quartz glass, CuS/PVP NWs film, and commercial Cs-doped WO3 film), thermocouple, and xenon light (detailed in Figure S8).5 The intensity of the xenon lamp that resembles the solar spectrum was 1 W/cm2. Without the heat source, the initial temperature was 24.5 oC for all cases. A plot in Figure 5d shows the temperature change as a function of irradiation time up to 2000 seconds. The indoor temperature with the quartz glass is rapidly elevated to 39.5 oC (∆T = 15.0 oC). For the commercial CWO film on glass, the temperature increases to 34.6 oC (∆T = 10.1 oC), while the indoor temperature with the CuS/PVP NWs film slowly increases (glass ∆T = 9.4 oC; PET ∆T = 9.7 oC; PEN ∆T = 9.9 oC). The experimental result shows a potential of the electrospun CuS/PVP NWs as an energy saving material for various thermal shielding applications.

Conclusion In this work, we introduce advanced materials processing and surface engineering technologies to manufacture the first example of electrospun CuS/PVP NWs on a flexible membrane. Experimental results show that the hybrid structure of CuS NPs-embedded PVP NWs has a consistent performance in transmittance at NIR and haze factor at a high temperature (85oC) and humidity condition (85%) as well as at cyclic bending and acceleration. The fabricated thermal shielding films from electrospinning demonstrate excellent NIR shielding efficiency (87.15%) from in-situ temperature monitoring in a simulated box. In addition, direct attachment of the NW film on the window of a model car for sunlight exposure shows about twice-higher shielding efficiency than a commercial CWO film. Collectively, this paper captures a potential of


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electrospinning-based manufacturing method of CuS/PVP NWs for energy saving, thermal shielding applications. A future work will focus on further improvement of the visible transmittance via a doping or control the morphology and size of the CuS NPs.

Experimental Section Materials. A copper plate electrode (2.5 cm × 5 cm × 0.1 cm), sodium sulfide nonahydrate (Na2S·9H2O, ≥98.0%), polyvinylpyrrolidone (PVP, Mw ~1,300,000), methyl ethyl ketone (C2H5COCH3, >99%) were purchased from Sigma-Aldrich. Ammonia water (NH4OH, 25 - 28%), formic acid (HCOOH, 99.5%), acetic acid (CH3COOH, >99%), sodium citrate dihydrate (C6H5Na3O7·2H2O, >99%), and ethanol (EtOH, 95%) were obtained from Dae-Jung Chemical. Novec EGC-1720 and fluorinert FC-40 were purchased from 3M. All reagents were used without further purification. Synthesis of CuS NPs. The CuS NPs were synthesized by a previously-reported process.19 High concentration Cu precursor were prepared using electrolysis method. A direct voltage of 40 V was applied between Cu plate electrodes in an electrolyte solution consisting of formic acid, acetic acid, and ammonia water. The Cu ion solution was evaporated to increase the Cu concentration to 15 wt%. To prepare the S precursor, sodium sulfide nonahydrate (1.25 M) and sodium citrate dihydrate (1 M) were dissolved in DI water (20 ml) at a reaction temperature of 70 oC. Subsequently, the Cu precursor (5 ml) was injected into the solution by a syringe pump with a rate of 1 ml/min. After injection, the reaction was maintained for 5 min. The resulting suspensions were washed and centrifuged four times with DI water. Fabrication of thermal shielding films. The thermal shielding films deposited with electrospun CuS/PVP NWs were fabricated on glass, antistatic-treated PET and PEN film. PET and PEN



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substrates were immersed in a solution composed of novec EGC-1720 and fluorinert FC-40 for the antistatic treatment. The substrates were then sonicated in ethanol to remove the residuals and dried in a stream of nitrogen gas. PVP polymer was dissolved in the CuS suspension that is the concentration of 1 wt.% in the solution, and then the solution was loaded into a plastic syringe with a 25-gauge needle. Conditions included a voltage of 25 kV, feeding rate of 0.3 ml/h, drum collector speed of 400 rpm, and distance of 15 cm. The electrospinning process was carried out at 40 oC in a relative humidity of 10%. To manufacture the thermal shielding films, methyl ethyl ketone was dropped and spin-coated on the electrospun CuS/PVP NWs. Characterization. The crystallinity and morphology of the synthesized NPs and NWs were observed by XRD (Ultima IV CuK, Rigaku), HRTEM (JEM-2100F, JEOL), and FE-SEM (MIRA3, Tescan). A UV-Vis-NIR spectrophotometer (UV-2600, JEOL; Jasco V670, ISN-723;) was used to confirm the thermal shielding effect. Light scattering though the film was measured using a haze meter (NDH 2000N, Nippon Denshoku Industries Co.) To compare the samples before and after the antistatic treatment, FT-IR (Nicolet iS10, Thermo Scientific) was used. A bending test was performed using homemade IPC sliding machine to investigate the flexibility of CuS/PVP NWs film. The samples were loaded into the bending system consisting of lower and upper plates. After the bending test of 5,000 and 10,000 cycles, the NIR shielding efficiency and haze were measured.


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Figure 1. Electrospun CuS/PVP NWs to fabricate thermal shielding films. (a) TEM image of CuS nanoparticles dispersed in aqueous solution before the electrospinning (Scale bar 5 nm). (b) Deposition mechanism between the CuS/PVP NWs and the fluorinated polymer surface. TEM images of (c) low (Scale bar 100 nm) and (d) high resolution (Scale bar 10 nm). (e) STEM image of CuS/PVP NWs (Scale bar 100 nm).



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Figure 2. Optical characterization of the thermal shielding film with the CuS/PVP NWs. (a) Optical images, (b) UV-Vis, (c) transmittance at 550 nm and haze spectra with respect to spinnin g time.


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Figure 3. CuS/PVP NWs electrospun on glass and fluorinated polymer surface. (a) FT-IR transmittance spectra of pristine PET, PEN films and the fluorinated PET, PEN substrates. FESEM images of CuS/PVP NWs on (b) glass, (c) PET, and (d) PEN (Scale bar 1 μm).



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Figure 4. Optical properties of fabricated thermal shielding films. Optical images of CuS/PVP NWs on (a) glass, (b) PEN, and (c) PET. d) UV-Vis transmittance of the films composed of CuS/PVP NWs, CuS NPs, and CWO under optimal condition. Change of NIR shielding efficiency (black) and haze factor (blue) of CuS/PVP NWs films upon e) cyclic bending and f) 85 oC - 85% test for 1 week.


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Figure 5. In situ temperature change monitoring of the thermal shielding film attached on the windows of model building and car. (a) Before and (b-c) after the coating of thermal shielding materials (b: commercial CWO, c: CuS/PVP NWs) on the model car windows, the comparison of the temperature differences in and outside the car. The light source is real sun spectrum. (d) The changes of the interior air temperature in the sealed box with the slide glass, CuS/PVP NWs on glass, PET, PEN film, and CWO on glass as a function of time under light irradiation of a xenon lamp.



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Table 1. Thermal shielding efficiency calculated in Figure 4d, transmittance at 550 nm, and haze factor.

CuS/PVP NWs on glass CuS/PVP NWs on PEN CuS/PVP NWs on PET CuS NPs on PET CWO on glass

SNIR (%)

Haze (%)

Suv (%)

Tvis (%)

85.57

1.90

90.84

67.30

85.42

1.95

97.60

60.56

87.15

2.01

90.12

59.75

78.99

2.94

88.59

59.86

85.23

2.28

62.75

58.84


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Note S1. The mechanism of the interaction between light and the film. Figure S1. XRD and UV-Vis analysis of CuS nanoparticles. Figure S2. Morphology of CuS/PVP nanowires. Figure S3. Histrodiagram of CuS/PVP nanowries. Figure S4. Optical properties. Figure S5. Cross-sectional FE-SEM view of CuS/PVP NWs. Figure S6. Reflectance spectra of the thermal shielding films. Figure S7. TEM images of the film fabricated with CuS NPs. Figure S8. Bending test. Figure S9. Schematic illustration of a simulated experiment. Table S1. Transmittance at 500 nm and Haze meter from Figure 2. Table S2. Optical values in NIR region.



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AUTHOR INFORMATION Corresponding Author *Prof. Yong-Ho Choa, #420, 5th Engineering Building, 55 Hanyangdaehak-ro, Ansan, Gyeonggido 15588, South Korea, [email protected] *Prof. Woon-Hong Yeo, Petit Microelectronics Center, 791 Atlantic Drive NW #204, Atlanta, GA 30332, USA, [email protected].

ORCID Yong-Ho Choa: 0000-0002-1254-3593 Woon-Hong Yeo: 0000-0002-5526-3882

Author Contributions Y.-T.K., W.-H.Y., and Y.-H.C. conceived and designed the research; Y.-T.K., S.H.R., and J.W.S performed the experiment and analyzed the data; Y.-T.K., W.-H.Y., and Y.-H.C. wrote the paper.

Acknowledgement We acknowledge a research fund from Hanyang University (HY-2018-N).

Conflict of Interest The authors declare no competing financial interest.


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PVP Nanowires and Superior Near-Infrared Filtration

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