[
    {
        "id": 1,
        "chunk": "# Highly transparent superhydrophilic graphene oxide coating for antifogging  \n\nXuebing Hu a,b,n, Yun Yu b,nn, Yong Wang b, Yongqing Wang a, Jianer Zhou a, Lixin Song b  \n\na Key Laboratory of Inorganic Membrane, Jingdezhen Ceramic Institute, Jingdezhen 333001, China b Key Laboratory of Inorganic Coating Materials, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 201800, China",
        "category": " Abstract"
    },
    {
        "id": 2,
        "chunk": "# a r t i c l e i n f o",
        "category": " Abstract"
    },
    {
        "id": 3,
        "chunk": "# a b s t r a c t  \n\nArticle history:   \nReceived 29 March 2016   \nReceived in revised form   \n28 June 2016   \nAccepted 29 June 2016   \nAvailable online 30 June 2016   \nKeywords:   \nGraphene oxide   \nFunctional   \nSuperhydrophilic   \nTransparent   \nAntifogging   \nSurfaces  \n\nThe superhydrophilic property of the surface allows water to spread completely across the surface rather than remain as droplets, thus making the surface antifogging. In this work, graphene oxide was prepared by the modified Hummers method, and the superhydrophilic and highly transparent functional graphene oxide coating had been fabricated on the glass substrate through a spin coating process. The as-prepared coated glass had a static water contact angle of $3.7^{\\circ}$ and a relatively high transmittance reaching about $76\\%$ throughout the visible region. For comparison, we studied the antifogging properties of the graphene oxide coated glass and the bare glass surfaces. The result shows these glass exhibits absolutely different fogging characteristics, and the graphene oxide coated glass has the superior antifogging property.  \n\n$\\circledcirc$ 2016 Elsevier B.V. All rights reserved.",
        "category": " Abstract"
    },
    {
        "id": 4,
        "chunk": "# 1. Introduction  \n\nThe wettability of solid surface is an attractive topic due to its importance in fundamental research and practical applications [1]. Water vapor can condense on solid surface at a certain temperature or humidity, and water will form little droplets on a solid surface if the surface is poor hydrophilic or hydrophobic. Therefore the light would be refracted and scattered by water droplets so that the transparent materials turn hazy, which causes fogging problem [2]. Endowing the solid surface with excellent wetting characteristic such as superhydrophilicity is a very efficient way to solve the above-mentioned problem [3]. Nowadays, superhydrophilic surface, a special wettability with a water contact angle of less than $5^{\\circ}$ , has received great attention as antifogging coating [4]. Numerous materials, for example, metal oxide $\\mathrm{TiO}_{2}$ , $z_{\\mathrm{{nO}}}$ , $\\mathsf{S n O}_{2}$ and ${\\mathsf{W O}}_{3}$ ) and graphene had been developed for preparing superhydrophilic surface [5–9].  \n\nFollowing the studies on graphene, graphene oxide (GO) has been widely investigated in recent decade, since its many unique and interesting properties such as large external surface area, excellent corrosion-resistant, good antibacterial property, and high mechanical strength, with the advantage of having a simple and inexpensive synthesis process [10,11]. Nowadays, GO based coating can be used for a wide range of applications such as corrosion protection, bacterial growth inhibition, water treatment and so on [12–14]. In particular, due to the presence of various oxygen containing functional groups such as epoxy, hydroxyl and carbonyl groups on its basal planes and edges, GO exhibits hydrophilic property and its wetting ability can be adjusted by the synthesis process [15–17]. However, it is difficult for common graphene oxide coating to be used as superhydrophilic surface material, since the microstructure and functional groups composition of GO coating can not be tailored easily. The previous preparation methods of superhydrophilic GO coating mostly included the complex step of reduction and bridization [18–20]. Therefore, a fast and facile approach for the high optical transmittance and superhydrophilic GO coating fabrication needs to be explored. Especially, due to the above-mentioned intriguing properties of GO, GO based coating has become a very competitive and promising candidate for anti-fogging application.  \n\nIn the current work, we present a simple and low-cost method for preparing high performance GO coating on the glass substrate. The coating exhibits superhydrophilicity, superior antifogging property and high optical transmittance throughout the visible region. Our work would greatly simplify the fabrication procedure of high performance GO coating and accelerate its promising applications in industry and daily life, such as mirrors, window glasses, windshields of automobiles, and so on.",
        "category": " Introduction"
    },
    {
        "id": 5,
        "chunk": "# 2. Experimental procedure",
        "category": " Materials and methods"
    },
    {
        "id": 6,
        "chunk": "# 2.1. Materials  \n\nMicrocrystalline graphite powders $99.9\\%$ purity) were purchased from Qingdao Sanyuan Graphite Co., Ltd. Potassium permanganate (AR), sodium nitrate (AR), concentrated sulfuric acid (AR, $98\\%$ ), hydrogen peroxide (AR, $30\\mathrm{wt\\%}$ aqueous solution), and quantitative filter paper were purchased from Sinopharm Chemical Reagent Co., Ltd.",
        "category": " Materials and methods"
    },
    {
        "id": 7,
        "chunk": "# 2.2. Synthesis of GO  \n\nGO was prepared using the modified Hummers method [15]. To remove the onus of oxidant and other inorganic impurity, the GO slurry was washed with the deionized water by repeated vacuum filtration through a $100\\mathrm{nm}$ cellulose acetate membrane. Then the suspension was centrifuged (13,000 rpm for $80\\mathrm{min}_{,}$ ), the supernatant was kept. Finally, the supernatant was diluted with deionized water to a total volume of $200\\mathrm{ml}$ in a $500\\mathrm{ml}$ flask to make a homogeneous GO suspension $(0.2\\mathrm{mg}1^{-1})_{\\cdot}$ ) for storing.",
        "category": " Materials and methods"
    },
    {
        "id": 8,
        "chunk": "# 2.3. Preparation of GO coating  \n\nGO coating was deposited on the glass substrate by a spin coating process. Initially, the substrate was ultrasonically cleaned in ethanol and deionized water. After cleaning, the substrate was dried at $60^{\\circ}\\mathsf C$ for $^{1\\mathrm{h}}$ . Then, the above-mentioned GO suspension was dropped on the glass substrate and spun at $500\\mathrm{rpm}$ for $10s$ and dried at $60^{\\circ}C$ for $^{3\\mathrm{~h~}}$ to enhance the mechanical property of the coating. The other side of glass substrate was operated by the same procedure. Finally, the both sides of glass substrate were covered with GO coating.",
        "category": " Materials and methods"
    },
    {
        "id": 9,
        "chunk": "# 2.4. Characterization and measurement methods  \n\nX-ray diffraction (XRD, D8 ADVANCE, Germany) was recorded on a D/max $2550\\mathrm{V}$ diffractometer with Cu Ka radiation $\\scriptstyle\\lambda=0.1542\\mathrm{nm}$ ). X-ray photoelectron spectroscopy (XPS, MICROLAB 310F, Thermo Scientific, UK) spectra were measured using Mg Ka as the exciting resource. The microstructure of the sample was investigated by Transmission electron microscopy (TEM, JEM200CX, Japan). The morphology of as-prepared GO coating was obtained by field emission scanning electron microscopy (FE-SEM, Hitachi SU8220, Japan). The root mean square (RMS) surface roughness of the GO coating was examined with an atomic force microscope (AFM, Bruker, Germany) operating in the tapping mode. The superhydrophilicity of the sample was evaluated with the water contact angle instrument (SL200B, China) by measuring the static contact angle of the deionized water droplet. The transmittance of the samples was carried out by UV–visible spectrophotometer (UV2310, China) at normal incidence in the wavelength between 300 and ${900}\\mathrm{nm}$ . For examination of antifogging property, a GO coated glass and a bare glass were cooled at ca. $-15^{\\circ}C$ for $^{3\\mathrm{h}}$ in a refrigerator, and then exposed to humid laboratory air (room temperature: $20{-}30^{\\circ}\\mathsf C$ , relative humidity: 20– $40\\%$ .",
        "category": " Materials and methods"
    },
    {
        "id": 10,
        "chunk": "# 3. Results and discussion",
        "category": " Results and discussion"
    },
    {
        "id": 11,
        "chunk": "# 3.1. XRD patter, XPS spectrum and microstructure of GO  \n\nThe XRD pattern of GO is shown in Fig. 1a. GO has a low intensity peak at $11.8^{\\circ}$ , which attributes to the (001) reflection of GO. The C 1s XPS spectrum of GO is presented in Fig. 1b. The spectrum is decomposed into three fitted peaks using a Gaussian function. A binding energy of $284.9\\mathrm{eV}$ indicates the existence of C–C sp2 bonds in GO, while $287.1\\ \\mathrm{eV}$ results from C–O bonds (epoxy and hydroxyl groups), and $288.5\\mathrm{eV}$ shows ${\\mathsf{C}}{=}0$ bonds (carbonyl) are formed during the oxidation process [21].  \n\nThe sp2 carbon (C–C) fraction can characterize the oxidation degree in GO, which can be estimated by dividing the area by C 1s peak area [17]. From Fig. 1b, the C–C fraction of GO is about $46.4\\%$ . The result shows there are fewer C–C groups in GO and also reflects GO has more oxygen-containing functional groups. With the increase of these groups, the hydrophilic property of GO will be enhanced [22].  \n\nThe microstructure of GO is inspected by TEM. From Fig. 1c, the average size of GO nanosheets is about $150\\mathrm{nm}$ and GO has excellent dispersibility.",
        "category": " Results and discussion"
    },
    {
        "id": 12,
        "chunk": "# 3.2. The surface morphology and wettability of GO coating  \n\nThe surface morphology of the GO coating was characterized by FE-SEM and the tapping mode AFM. As shown in Fig. 2a, it indicates some aggregations have been formed due to the larger amount of GO loading, so that the coating surface is not smooth. The cross-section image of GO coating is shown in Fig. 2b, which indicates the thickness of GO coating is about $100\\mathrm{nm}$ As displayed in Fig. 2c, it is also observed the surface of the coating presents some roughness that possibly arises from the random overlap and aggregation of individual GO sheets. The RMS roughness ups to $9.32\\mathrm{nm}$ .  \n\nFrom Fig. 2d, the static water contact angle (WCA) of $\\sim3.7^{\\circ}$ is observed for GO coating. The small water contact angle suggests the coating represents the superhydrophilic property. From Fig. 2c, we can know the coating surface has a higher surface roughness. According to Wenzel equation [23]:  \n\n![](images/49c8500cafe5a50b4422ae51061ad3ddfbe9db0af3489162532361b0f4995eea.jpg)  \nFig. 1. (a) XRD pattern, (b) XPS spectrum and (c) TEM image of GO.  \n\n![](images/fb03128470b94a57f992d17952ace1a51900f7c0dba6f0e476286025523f3e1d.jpg)  \nFig. 2. FE-SEM images in (a) top view and (b) cross-sectional view of a GO coating, (c) three-dimensional AFM image, (d) water contact angle of GO coating on the glass substrate.  \n\n$$\n\\mathrm{cos\\theta_{w}}=\\gamma\\mathrm{cos\\theta_{e}}\n$$  \n\nwhere $\\uptheta_{\\mathrm{w}}$ is the apparent WCA in Wenzel state, $\\boldsymbol{\\upgamma}$ is the surface roughness factor and $\\uptheta_{\\mathrm{e}}$ is the equilibrium WCA on the horizontal and smooth surface. In the case of a principally hydrophilic surface, a decrease of WCA is predicted following an increase of $\\boldsymbol{\\upgamma}$ .  \n\nAs shown in Fig. 2c, it is clear that there are some corrugations on the coating surface and the RMS surface roughness is $9.32\\mathrm{nm}$ . The results indicate the coating has a higher surface roughness so that the value of $\\boldsymbol{\\upgamma}$ increases, which is beneficial to magnify the intrinsic wetting characteristic of the surface. It has also been further confirmed by Fig. 2d.",
        "category": " Results and discussion"
    },
    {
        "id": 13,
        "chunk": "# 3.3. The transmittance of GO coating  \n\nTo evaluate the optical transmission, a comparative test about the transmittance of the bare and GO coated glasses was carried out by UV–visible measurement.  \n\nAccording to the UV–visible spectra in Fig. 3, the GO coated glass shows a high transmittance reaching about $76\\%$ , while the transmittance of the bare glass is about $90\\%$ throughout the visible region. It is observed there was a decrease in transmittance with the GO coated, which can be ascribed to the light reflecting and scattering resulting from the surface roughness and the air-coating and the coating-substrate interfaces [24].  \n\nGenerally, the high roughness and high transmittance is a pair of competitive properties due to extensive light scattering [25]. Interestingly, it can be revealed from the above results of experiment that the GO coated glass has a good transmittance with the  \n\n![](images/ec5071836aee722bf0997f578b0b059235083143aeebbe6cd4b04b7cdbaa8db0.jpg)  \nFig. 3. UV–Vis transmission spectra of GO coated glass and bare glass.  \n\nRMS surface roughness ups to $9.32\\:\\mathrm{nm}$ .",
        "category": " Results and discussion"
    },
    {
        "id": 14,
        "chunk": "# 3.4. The antifogging of GO coating  \n\nWe investigated the antifogging properties of the GO coated glass and the bare glass, as shown in Fig. 4. Both glass samples were cooled at ca. $-15^{\\circ}C$ for $^{3\\mathrm{h}}$ in a refrigerator, and then simultaneously exposed to humid laboratory air.  \n\nAs shown in Fig. 4, the bare glass (left) fogged immediately and presented a large amount of tiny condensed droplets causing a significant reduction of the optical transmittance. As a comparison, the GO coated glass (right) remained clear and excellent transparency during the whole antifogging test. Thus, the GO coating should play an active role in antifogging property of glass. This special antifogging ability should be attributed to that the nearly instantaneous spreading of water droplets on the superhydrophilic surface. Thus, water could evaporate soon and the GO coated glass surface was kept clear at all times.  \n\n![](images/8aa7a980079defb554bfe672b93e14f8a2a478dd33ff8b61941ff1d9f69077ef.jpg)  \nFig. 4. Images of the fogging comparison experiment between GO coated glass and bare glass after cooling.",
        "category": " Results and discussion"
    },
    {
        "id": 15,
        "chunk": "# 4. Conclusion  \n\nWe presented a facile route to fabricate highly transparent superhydrophilic GO coating on glass substrate. The resultant superhydrophilic surface shows a static water contact angle of $3.7^{\\circ}$ and a transmittance reaching about $76\\%$ throughout the visible region. We also discussed the antifogging characteristic of GO coated glass and bare glass, the result demonstrates the GO coated glass has a superior antifogging property.",
        "category": " Conclusions"
    },
    {
        "id": 16,
        "chunk": "# Acknowledgements  \n\nThe authors gratefully acknowledge the support of this research by the National Natural Science Foundation of China (Grant No. 51262012) and the Foundation of Jiangxi Science and Technology Committee (Grant Nos. 20133ACB20007 and 20161ACB21008). The project also was funded by the Key Laboratory of Inorganic Coating Materials, Chinese Academy of Sciences (Grant No. KLICM-2014-07).",
        "category": " References"
    },
    {
        "id": 17,
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        "category": " References"
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]