Roll-to-roll graphene oxide radon barrier membranes

Journal of Hazardous Materials 383 (2020) 121148

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Roll-to-roll graphene oxide radon barrier membranes a,⁎

a

a

T b

c

Myoung-Seon Gong , Jae-Ryung Cha , Suk Min Hong , Cheolmin Lee , Dong Hyun Lee , ⁎ Sang-Woo Jood, a

Department of Nanobiomedical Science and BK21 PLUS NBM Global Research Center, Dankook University, Cheonan 31116, Republic of Korea Department of Chemical & Biological Engineering, Seokyeong University, Seoul 02713, Republic of Korea Consulting & Technology for Environment Health and Safety, Seoul 04788, Republic of Korea d Department of Information Communication, Materials Engineering, Chemistry Convergence Technology, Soongsil University, Seoul 06978, Republic of Korea b c

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Editor: Danmeng Shuai

Graphene oxide as a radon barrier in living environments was introduced by intercalating the polymer resincoated layer inside a multilayer membrane with an area of 1 × 10 m and a thickness of 2.5 mm, prepared by the roll-to-roll method. A 5 μm-thick graphene oxide polymer resin (GOPR) layer was coated on polyethylene terephthalate (PET) film (100 μm) between the two styrene-butadiene-styrene (SBS)-modified bitumen asphalt layers fitted for construction sites. The inserted graphene oxide materials were characterized by means of infrared, Raman, and X-ray photoelectron spectroscopy (XPS). Dispersion-corrected density functional theory (DFT) calculations suggested weaker binding energies on the oxide surfaces and higher penetration energy barriers of graphene nanopores for radon (222Rn) than in the cases of the atmospheric gas molecules Ar, H2O, CO2, H2, O2, and N2. Theoretical calculations of the graphene nanopores supported the higher barrier energies of 222 Rn than most ambient gases. The roll-to-roll prepared graphene materials exhibited good barrier properties for 222Rn as well as for the ambient gases. The purpose of our experimental and theoretical study is to provide a better understanding of using graphene-based materials to reduce the risk of carcinogenic radon gas in construction sites and residential buildings for practical applications.

Keywords: Radon barrier Roll-to-roll Density functional theory Polymer resin Graphene oxide

⁎

Corresponding authors. E-mail addresses: [email protected] (M.-S. Gong), [email protected] (S.-W. Joo).

https://doi.org/10.1016/j.jhazmat.2019.121148 Received 28 January 2019; Received in revised form 14 August 2019; Accepted 2 September 2019 Available online 04 September 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.

Journal of Hazardous Materials 383 (2020) 121148

M.-S. Gong, et al.

1. Introduction

2. Materials and methods

Recently, the nanocomposite barrier properties of hazardous species have been studied by the materials community (Gadipelli and Guo, 2015; Campos et al., 2019; Bera et al., 2018; Ghaemi and Safari, 2018). Theoretical modeling has been helpful to predict the adsorption properties of a radioactive gas on functional materials (Kong et al., 2019). Roll-to-roll graphene-based materials have seen considerable attention due to their versatile applications in many industrial areas (Novoselov et al., 2012; Zhang et al., 2017a; Bai et al., 2018; Ning et al., 2017). Chemical vapor deposition roll-to-roll graphene has a limited size of approximately 30 in. to aim mainly at electronic devices (Hesjedal, 2011; Bae et al., 2010; Deng et al., 2018). Membranes, microspheres, and nanocomposites have been introduced to the polymeric membrane materials to remove the water contaminants including heavy metal and fluoride ions (Sun et al., 2018; Zhang et al., 2017b; Zhang et al., 2019). Considering that gas barrier properties are needed in construction sites and residential buildings (Groves-Kirkby et al., 2006; Jelle, 2012; Cosma et al., 2015), polymer membranes on a scale larger than a few meters were introduced for the installation of barriers using a roll-to-roll process (Ye et al., 2018; Kidambi et al., 2018; Chandrashekar et al., 2015; Guo et al., 2017). Graphene-containing polymer resins have been introduced in roll-totoll materials (Kuilla et al., 2010; Li et al., 2016). Graphene nanocomposites of resin materials (Singh et al., 2018; Kim et al., 2010) have been studied including the PVA/GO due to its excellent gas barrier properties (Kim et al., 2011; Layek et al., 2014). Radon (222Rn) is a naturally existing radioactive noble gas exhibiting alpha decay with a half-life of 3.8 days (Zeng et al., 2019) As a tracer of geophysical processes and a potential threat to environments (Schuber et al., 2012; Werzi, 2010; Liu et al., 2018; Zhu et al., 2018; Kligerman and White, 2011; Schubert et al., 2012; Poncela et al., 2004), radon, a well-established risk factor for human lung cancer, is present at low concentrations on the ground floors of most buildings. Residential 222 Rn decay products can also cause lung cancer (Meisenberg et al., 2017; Field et al., 2017). Recently, the radioactive noble gas 222Rn and its progeny have recently come to be considered a health risk in the indoor environment because of their contribution to radiation doses in the lungs and their potential for inducing lung cancer (Krewski et al., 2005; Branion-Calles et al., 2016; Yan et al., 2017). Regarding the risks to the human health, the recommended radon concentration in the United States for indoor air quality regulation in public use facility is 4 pCi/L (Sethi et al., 2012). These risks have prompted the introduction of barrier membranes such as polymer and Si-based radon barrier membranes (Daoud and Renken, 2001; Hofmann et al., 2011; Zhong et al., 2019; Khan et al., 2019; Jiránek and Kačmaříková, 2019, Maes et al., 2018; Maier et al., 2018). There have several experimental and theoretical reports of their gasbarrier properties with environmental gases such as H2, He, N2, H2O, CO2, CH4, and Hg (Bunch et al., 2008; Su et al., 2014; Guo et al., 2012; Chi et al., 2016; Drahushuk et al., 2016). Quantum mechanical calculations can provide the interactions of the gaseous molecules on the surfaces of graphene, along with the penetration barrier energies (Lalitha and Lakshmipathi, 2017; Schrier, 2010; Ambrosetti and Silvestrelli, 2014). Due to its heavy atomic weight, there have been limited reports of theoretical study on Rn. In this work, we examined a large area of graphene oxide polymer resin (GOPR) membranes to develop roll-to-roll-based graphene-inserted membranes. Theoretical calculations were used to explain the barrier properties of radon. Despite the previous reports of GO/PVA materials ; Layek et al., 2014) and randon barrier membranes (Daoud and Renken, 2001; Hofmann et al., 2011; Zhong et al., 2019; Khan et al., 2019; Jiránek and Kačmaříková, 2019; Maes et al., 2018; Maier et al., 2018), our study is the first implementation of GO-based materials in the construction sites of a building with a large area of 1 × 10 m.

2.1. Chemicals and preparation of radon barrier films Graphene oxide (GO) in aqueous solution was synthesized by reacting graphite with H2SO4 as described in the literature method (Kim et al., 2011). Polyvinyl alcohol (PVA) (86.5–89.0 mol% hydrolyzed, Mw = 22,000 g/mol) was purchased from Junsei Co. (Tokyo, Japan). PET film with a thickness of 100 μm was obtained from Sunkyong Chemicals (Seoul, Korea). Styrene-butadiene-styrene (SBS) modified bitumen sheets were purchased from RAVI Co. (Eumseong, Korea). PVA (10.0 g) was dissolved in deionized water (D.W, 90.0 g) at 70 °C, and GO (10 mg) in aqueous solution (20 mL) was added to the PVA solution (Li et al., 2016). The PVA/GO solution was rolled at 100 rpm using a roll milling machine at 25 °C for 24 h. The surface of the PET film was washed ultrasonically in ethanol and distilled water for 1 min to improve wettability of the PVA/GO solution. The PVA/GO solution was coated onto a PET film with a thickness of 50 μm by an automatic bar coater at 100 °C and then dried for 10 min. The GO/PVA/PET composite film was dried again in a vacuum oven at 90 °C for 6 h. SBS modified bitumen sheets were coated on the upper and lower layers of the GO/PVA/PET film using a lamination machine at 60 °C. Asphalt has been introduced to construction sites (Yildirim, 2007) with modified bitumen SBS polymer (Kok and Colak, 2011; Yu et al., 2007; Zehtabchi et al., 2018; Wu et al., 2009). This procedure is illustrated in Fig. 1. This depicts a roll-to-roll scheme of graphene-inserted Radon barrier membranes along with stacked layers of GOPR-inserted radon barrier membrane. 2.2. Spectral and gas permeation measurements Fourier transformation infrared (FT-IR) spectroscopy was conducted using a Nicolet 6700 equipment. Optical images were collected using a Leica microscope (Lens 20x). Raman spectra were obtained using a Thermo DXR2xi microscope under an excitation wavelength of 532 nm. X-ray photoelectron spectroscopy (XPS) spectrum was obtained by the electron spectroscopy for chemical analysis (ESCA) method using a Sigma Probe (ThermoVG, UK) under vacuum conditions of 7 × 10−9 mB in the presence of X-rays, and the absence of ions, using an etching flood gun with a monochromatic Al-K X-ray source (15 kV, 100 W, 400 μm). The spectral position was calibrated by the C1s (284.5 eV) peak. The spectrum was obtained at a wide scan of 50 eV with a step size of 1.0 eV, and a narrow scan pass of 20 eV with a step size 0.1 eV, using Avantage (ThermoVG) software. Cross-sectional field emissionscanning electron microscopy (FE-SEM) and energy dispersive X-ray spectrometry (EDS) data were obtained using a JEOL 7800 microscope. O2 gas measurements were performed using an Illinois 8003 (Systech, USA) and following ASTM F1927, whereas CO2, N2, and H2O were checked by a Toyoseiki Model BT-3 device according to KSM ISO 2556. The measurement of the radon diffusion coefficient of the GOPRinserted membrane was performed in accordance with the requirements for determination of the radon diffusion coefficient stated in the K124/ 02/95 test method. H2O, CO2, O2, and N2 penetration properties were tested from Korean Conformity Laboratory (Seoul, Korea), while radon barriers were tested by Czech Technical University (Prague, Czech Republic). Radon barrier samples were 160 mm in diameter with a thickness of 2.42 mm. Radon diffusion coefficient was measured according to the accredited method K124/02/95 (method C of ISO/TS 11665-13). The tested sample was placed between two containers. Radon diffuses from the lower repository, which was linked to the radon source, through the sample to the upper repository. When the steady state concentration profile within the sample was reached, the growth of radon concentration in the upper container was measured. From the known time dependent curve of the radon concentration increase in the upper container the radon diffusion coefficient can be calculated. This test method was approved by the State Office for 2

Journal of Hazardous Materials 383 (2020) 121148

M.-S. Gong, et al.

Fig. 1. Roll-to-roll scheme of graphene-inserted radon barrier membranes. Stacked layers of GOPR-inserted barrier membrane. The thicknesses of graphene and PET layers were 5 μm and 100 μm, respectively. Effective removal of radon in a residential building.

2.3. Theoretical calculations

Nuclear Safety on 6.8.1998. For graphene-inserted material, the steadystate radon concentration in the lower container was 47.7 ± 0.6 MBq/ m3. The radon supply rate into the upper container was 0.5 ± 0.2 Bq/ m3s. The measuring device was a radon monitor RDA 200(N12) micrometer (N11) at laboratory temperature of 21 ± 2 °C and relative humidity of air in the laboratory of 38% ± 4%. The pressure difference between the lower and upper containers was 0 Pa.

We carried out density functional theory (DFT) calculations (Tran et al., 2018) using the Gaussian 09 program (Nam et al., 2018). Binding energies of seven gases were calculated on the epoxide, OH, and COOH groups on graphene oxides at the level of M062X/CEP-31G considering the dispersion effects. The A-type, B-type, and C-type pores were approached on the Perdew, Burke, and Ernzerho (PBE) functional, in turn based on the CEP-31G functional basis sets. In line with previous calculations (Lalitha and Lakshmipathi, 2017), energy barrier energies of 3

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previous studies Zhang et al., 2010; Alhosseini et al., 2012). We could only characterize the XPS spectra of GOs prior to inserting the top thick resin, because of how thin the layers of GOs were compared with those of the other layers. Despite the independent XPS characterization, the prominent spectral peaks, along with the cross-sectional SEM images, indicate the presence of thin GO layers between the other thick layers. The FT-IR spectrum of GO displaced several peaks, at 1091, 1432, 1738, 2913, and 3321 cm−1, which are associated with stretching of CeO, CeC, C]O, CeH, and OeH functionalities. This indicated the presence of oxy-functional groups binding on a carbon matrix of GO. The Raman measurements of PVA/GOs, represented two peaks at 1346 and 1604 cm−1 which attributed to D-band and G-band. The G-band per-formed the presence of sp2 carbon atoms, and the D-band assigned to the existence of sp3 carbon molecules. This was indicated through the ratio ID/IG was merely 1.206. The intensity ratios of these peaks (ID/IG) were due to the presence of oxy-groups on surface of GO. After GO was reduced, these groups disappeared. As a consequence, the amount of sp3 carbon molecules decreased and the one of sp2 carbon molecules increased. XPS spectrum revealed strong peaks of C1s and O1s. In an expanded view of C1s region of XPS spectrum, there are four different features maximized at 285, 286, 287, and 289 eV, corresponding to C]C/CeC in aromatic rings, CeO (epoxy and alkoxy), C]O, and COOH groups, respectively. The assignments of C1s and O1s components indicated the presence of the particular oxygen functional groups. In addition to the sp2 graphite component at 285 eV, four broad components could be found to account for the multiple overlapping C1s features. The component at 286 eV is attributed to epoxide group (CeOeC), whereas the smaller components at 287 and 289 eV are related to carbonyl (> C]O) and carboxyl groups (COOH or HOeC]O). The assignments were in agreement with the literature (Zhang et al., 2010).

seven gases were estimated for the three different types of graphene nanopores at the level of PBEPBE/CEP-31G. Nanopores were prepared according to the procedure (Ambrosetti and Silvestrelli, 2014). Permeation energy barriers could be estimated by comparing the total interaction energy of the gas molecule with that from its most stable adsorption configuration of the molecule above the pore. For the z coordinate of its center of mass, the energy Ez0 can be defined as that at the same height of the graphene plane (z0). These could be estimated from ab initio DFT calculations. The ΔE values were tested for the seven gases of H2, O2, H2O, CO2, N2, Ar, and Rn. The molecular dynamics simulations (MD) have been carried out using atom centered density matrix propagation (ADMP) at 350 K. 3. Results and discussion 3.1. Preparations of GOPR-inserted radon barrier membrane Fig. 1 shows our roll-to-roll scheme of graphene-inserted radon barrier membranes to illustrate the stacked layers of the GOPR-inserted barrier membrane. The thicknesses of the graphene and PET layers were 5 μm and 100 μm, respectively. Figure S1 shows roll-to-roll radon barrier membranes (1 m x 10 m x 2.5 mm). We admitted that one of the biggest challenges of the roll-to-roll method is inhomogeneity or defects during the process of mechanical rolling. Under our preparation conditions, the tolerance of a thickness of 2.5 mm is estimated to be as low as 0.03 mm. After its sampling, the top view revealed the silica sand with a mechanical strength to be suitable for the construction sites. The side view indicated the thickness for the barrier in building. FE-SEM images were obtained to better understand the structures. 3.2. Physical characterization Fig. 2 depicts a cross-sectional FE-SEM images of roll-to-roll GOPRinserted radon barrier membranes. EDS mapping analysis indicated the carbon and oxygen layers of the thickness of ˜5 μm. Physical characterizations of inserted GO and PVA are illustrated in Fig. 3. The infrared and XPS spectral assignments have been performed from the

3.3. Ambient gas barrier As illustrated in Table 1, graphene barrier membrane showed good barrier properties for Rn as well as. O2, CO2, N2, and H2O. In order to check the performance, we compared the 15 μm aluminum-inserted Fig. 2. (a) Cross-sectional SEM image showing the silica sand, SBS-asphalt, and 100 μm PET layers (left). An expanded view showed the 4.7 μm GOPR layers. (b) EDS mapping of carbon and oxygen for GOPR-inserted radon barrier membranes suggested graphene oxides layers with the different composition of the other layers. The red and green colors indicate the color-coded energy dispersive X-ray mapping data (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. 3. (a) Infrared and (b) Raman spectra of graphene oxide-based materials. (c) XPS spectra of graphene materials for the C1s and O1s region and (d) an expanded view of the C1s expanded region.

membrane with the 360 μm woven fabric. It was found that both the membranes showed the enhanced gas barrier properties as well, particularly H2O. Considering the detection limit, the prepared membrane exhibited the barrier properties. It was found that the graphene-inserted mem-brane showed better O2 barrier value of 0.3 cm3/(m2 day) than that of aluminum of 3 cm3/(m2 day).

which for the normal distribution corresponds to the probability of coverage approximately 95%. The values are summarized in Table 1. It is noteworthy that GOPR-inserted membrane exhibited better Rn barrier property than those of the aluminum-inserted PET membrane. The diffusion coefficients of PET-alumina and PET-graphene were found to be < 5.7 × 10−12 and < 2.2 × 10−12 m2/s, respectively. These values were discovered to be quite competitive among 29 waterproof polymer materials except MB-Al-PE (membranes combining SBS modified bitumen and Al and PE), EVOH (ethylene vinyl alcohol), and PP (polypropylene) carrier film layer in the recent report (Jiránek and Kačmaříková, 2019), which demonstrates the effectiveness of our membrane system. We also conducted an additional evaluation of removal performance. According to the preliminary data through experiments using Eperm, RAD7, and FRD400 portable radon measurement devices, the concentrations of radon could be lowered by approximately 81.9% using our barrier membrane for the two repetitive tests. We plan to continue to conduct additional on-site radon measurements and analysis.

3.4. Radon barrier measurements To compare the radon barrier property, we measured radon diffusion coefficients. Using the determined value of the radon diffusion coefficient D, the following parameters of the waterproofing material can be calculated as in the supporting information section. Radon diffusion length, resistance, and transmittance in the material were determined to be 1.02 × 10−3 m, 2453 Ms/m, and 4.08 × 10-10 m/s, where λ = 2.1 × 10-6 s-1 is the radon decay constant and d is the thickness of the material [m]. The measured values of aluminum and graphene-inserted membranes were found to be < 5.7 × 10-12 and < 2.2 × 10-12 (m2/s). This value was within the recommended limit of the Rn barrier products (Daoud and Renken, 2001; Hofmann et al., 2011; Zhong et al., 2019; Khan et al., 2019; Jiránek and Kačmaříková, 2019; Maes et al., 2018; Maier et al., 2018). Table 1 shows relative permeability ratios of O2, CO2, N2, H2O, and Rn for PET, PET-aluminum, and graphene with the units of cm3/(m2·day), cm3/ (m2·day·atm), cm3/(m2·day·atm), and 10 × 10-12 m2/s, respectively. The stated measurement is the uncertainty with the coefficient k = 2,

3.5. Theoretical calculations To explain radon barrier properties of GO-based materials, we performed DFT calculations. As illustrated in Fig. 4, we attempted to estimate the binding energies of the epoxide, OH, and COOH groups of GO surfaces by mean of DFT calculations. As summarized in Table 2, Rn

Table 1 Comparison of gas barrier properties of ambient gases with radon. Materials

O2 cm3/(m2·day)a

CO2 cm3/(m2·day·atm)b

N2 cm3/(m2·day·atm)b

H2O (g/m2·24 h)c

Radon (m2/s)

PET a PET + Aluminum PET + Graphene

18 3.0 0.3

52 < 3.0 < 3.0

3.3 < 3.0 < 3.0

5.5 < 0.1 3.1

– < 5.7 × 10−12 < 2.2 ⅹ 10−12

a b c

Measured from Korea conformity laboratory. Detection limit: 3.0 cm3/(m2·day·atm). (38 ± 2)℃ 100% R.H. 5

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Fig. 4. (a) Rn on the epoxide, OH, COOH groups on GO surfaces. (b) A-type (left), B-type (middle), and C-type (right) nanopores. (c) Interaction energies between H2 and Rn as a function of adsorption height in the C-type nanopore. Table 2 Estimated binding energies of seven gases on the epoxide, OH, and COOH groups on graphene oxides.

Table 3 Estimated energy barriers of seven gases for the three type of graphene nanopores.a

Gas

Carbon

Epoxide

OH

COOH

Gas

A-type pore

B-type pore

C-type pore

H2O CO2 H2 O2 N2 Ar Rn

−4.62 −5.18 −1.32 −8.43 −2.58 −1.52 −2.66

−8.75 −7.36 −1.44 −4.79 −3.45 −1.69 −2.97

−14.08 −9.08 −2.31 −42.03 −4.17 −1.67 −2.83

−19.98 −6.60 −1.15 −41.1 −4.29 −2.62 −0.484

H2O CO2 H2 O2 N2 Ar Rn

25.77 53.45 13.38 26.29 70.56 82.53 219.54

23.57 46.14 7.57 9.14 28.78 22.74 58.14

9.27 28.74 1.65 3.78 9.93 3.62 12.34

a Unit in kcal/mol. The energies were calculated at the level of M062X/CEP31G.

a Unit in kcal/mol. The energies were calculated at the level of PBEPBE/CEP31G.

is predicted to bind weakly on the GO surfaces than the ambient gases of H2O, CO2, and O2. In particular, Rn would bind most weakly on the COOH group. As in the case of H2 and N2, Rn may weakly interact the GO surfaces due to their weak binding energies at the level of M062X/

CEP-31G after considering the weak dispersion energies. To estimate the barrier properties, we introduce the three types of graphene nanopores as depicted in Fig. 4. As summarized in Table 3, Rn would have the highest energy barriers for A and B type nanopores. The

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move quite far away from the oxygen surfaces due to their weak bindings, it is not expected that Rn can closely approach the GO surfaces. It was found that O2 may bind strongly on the C]O and COOH groups of the GO surfaces. O2 would adsorb on the GO surfaces and stick without penetration. This may explain why the graphene-inserted membrane exhibited good barrier properties for O2. Our proposed mechanisms for the graphene radon barrier suggest that there are two factors that affect the radon barrier: (i) adsorption on the oxygen groups on graphene oxide surfaces and (ii) nanopore penetrations.

barrier energies are estimated from the energy differences of the optimized geometries and zo positions at the same graphene plane. Rn atoms are expected to have higher energy barrier than H2O, CO2, O2, N2, H2 and Ar for A-type and B-type nanopores as summarized in Table 3. In the case of C-type nanopores, H2O, O2, N2, H2, and Ar are expected to have higher barrier energy than that of Rn. Among them the energy barriers of the C-type nanopores for H2, H2O, and Rn were estimated to be 1.65, 9.27, and 12.34 kcal/mol. As depicted in Fig. 4, we calculated the energies at the adsorption heights up to 5 Å from the graphene plane with the interval distance of 0.5 Å. It was found that H2O and O2, have relatively low energy plateau of ˜1.1 and ˜3 kcal/mol, respectively at the distance of 5 Å from the Ctype nanopore graphene plane. For both H2O and O2, the most stable energy was found at 1 Å away from the plane. Their energy differences from the graphene pore were estimated to be only ˜0.1 kcal/mol. On the other hand, Rn appeared to have higher energy barrier of ˜8 kcal/mol at the pore with respect to the most stable distance of ˜3 Å. This result suggests that Rn should have higher barrier properties at the graphene nanopore in comparison with H2O and O2. Our DFT calculations can be matched with the steady-state radon concentration and diffusion coefficient measurements according to the accredited method K124/02/95 (method C of ISO/TS 11665-13) to exhibit the Rn barrier property of the GOPR-inserted membrane. On the basis of these DFT results, we performed MD calculations. As shown in, Fig. 5. H2 seems to freely move around the nanopores of graphene, whereas the distance of Rn turns out to keep moving far away from the graphene plane.

3.7. Prospects of practical applications Our radon barrier system appeared to block the radon concentrations at an on-site construction site by 60–90%. according to portable radon measurement devices, as mentioned. The reduction percentages for the three independent measurements were found to be 66.7, 90.2, and 88.0%, respectively. Considering that graphene oxide membranes were inserted in the membrane, such high blocking gas barrier properties could be understood according to the proposed mechanisms of the weak binding of radon atoms on the graphene oxide surface, functionalized groups, and inefficient penetrations in the nanopores. Our gas barrier system could be practically applied to building materials to block hazardous gases from the soil. Due to the roll-to-roll processes used to combine the silica sand, SBS asphalt bitumen, and PET, the blocking system can be installed in residential buildings, such as country houses and army barracks. EHS Korea is currently working on practical usages of the developed system as building materials. After the introduction of the barrier membrane into a new building, we found that the average values of radon became considerably decreased to 28.9, 43.9, and 22.4 Bq/m3 using the E-perm, FRD400, and RD200 portable devices, respectively, below the maximum permissible value of 148 Bq/m3 to demonstrate the prospect of practical applications.

3.6. Proposed mechanisms Considering that GOs were introduced, we estimated the interaction energies of the ambient gases on the graphene oxide layer. Table 2 illustrates the estimated binding energies of seven gases on the epoxide, OH, and COOH groups on GO surfaces. It was found that radon atoms bind more weakly on the oxygen groups than the ambient gases of H2O, CO2, O2, and N2. The binding energies of radon on the epoxide and OH groups were comparable to those of H2 and Ar. Such weak interactions of Rn may affect the gas-barrier properties of graphene-based materials. Since Rn atoms are expected to tend to

4. Conclusions A large area of graphene-based radon barrier membrane is reported in the present work. Using the roll-to-roll method, a graphene materialinserted multilayer membrane with a dimension of

Fig. 5. Snapshot of H2 (left) and Rn (right) diffusing through the C-type pore according to the ADMP simulations. Trajectories of H2 and Rn at 350 K with the interval of 1 fs until 1.5 ps. 7

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1 m × 10 m × 2.5 mm was prepared for the radon barrier. 5 μm-thick graphene resin materials were inserted be-tween PET and SBS-modified bitumen asphalt layers. We characterized the inserted graphene materials by means of infrared, Raman, and X-ray photoelectron spectroscopy (XPS). The roll-to-roll prepared graphene materials exhibited good barrier properties for both radon and ambient gases. To explain the gas-barrier properties, DFT calculations predicted weak binding energies on the oxide surfaces and low penetration energy barriers of graphene nanopores for radon.

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