Cold Wall Chemical Vapor Deposition Graphene-Based Conductive

Mar 20, 2018 - In this paper, we report on a continuous, flexible, and transparent graphene film obtained by cold wall chemical vapor deposition (CVD)...
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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Cold Wall Chemical Vapor Deposition Graphene-Based Conductive Tunable Film Barrier Maria Sarno,*,†,‡ Gabriella Rossi,† Claudia Cirillo,†,‡ and Loredana Incarnato†,‡ †

Department of Industrial Engineering, University of Salerno, via Giovanni Paolo II, 132, 84084 Fisciano, SA, Italy NANO_MATES Research Centre, University of Salerno, via Giovanni Paolo II, 132, 84084 Fisciano, SA, Italy

‡

S Supporting Information *

ABSTRACT: In this paper, we report on a continuous, flexible, and transparent graphene film obtained by cold wall chemical vapor deposition (CVD) on Cu foil. The good continuity of the graphene film, obtained in optimized conditions (e.g., pretreatment to increase copper grain size, Ar flow during synthesis to control Cu sublimation) was successfully transferred on a PET substrate to be applied with a double function of window electrode and barrier film. PET film after a single layer graphene deposition showed good performance: a sheet resistance of 0.6 kΩ/sq, a low reduction of transmittance in comparison with the bare polymer (only ∼3% in a large range), an increment of 95% for the oxygen barrier properties, and a very low water vapor transmission rate (WVTR) (∼96% reduction respect to PET substrate). This shows that graphene film may be an important alternative to conventional transparent electrode materials. We also speculate on the possibility to modulate polymer permeability at the nanoscale just applying a controlled-nanoporosity graphene layer. consumption are kept high.28 Moreover, they often require high amounts of hydrogen, which is in turn associated with safety issues.29 On the other hand, CVD on metal substrates has been the object of intense investigation, and progress has been made.30−39 However, although it is considered the most promising technique to synthesize graphene, many issues need to be addressed for industrial application, for example, the cost/ performance ratio.40,41 The cold wall CVD system,28,42 to date not as much explored, requires short times and lower energy consumption. A cost reduction of ∼99% has been estimated although the syntheses have been performed in ultrahigh vacuum (UHV).28 Furthermore, it offers favorable cooling conditions where fast cooling is crucial for the formation of continuous graphene. On the other hand, the materials from different laboratories are hardly reproducible, and therefore quality graphene is difficult to obtain. This is because of intrinsic characteristics of CVD endothermic processes, which are affected by (i) small deviations of composition and temperature, (ii) gas purity, (iii) catalytic support specificity, and (iv) need to have all the process information for reproducibility. This is even more crucial in the case of cold wall CVD due to the delicate heat supply to the copper foil surfaces and the shorter synthesis times. Here we report all the necessary details for graphene preparation by cold wall CVD

1. INTRODUCTION Recent developments in nanoscale materials have focused on nanocomposites with unique properties for various applications.1,2 Graphene films may be considered an effective alternative as window electrode, for example, in replacement of indium tin oxide (ITO) or fluorine tin oxide (FTO), because of their excellent transmittance and electrical properties.3 On the other hand, graphene is recently attracting great attention not only for its outstanding electrical,4 optical, and mechanical properties,5,6 but also for its gas barrier properties,7 because it can significantly reduce gas permeability when added to polymer substrates. Inorganic materials have been predominantly employed as gas barrier films.8−10 However, poor mechanical flexibility and complicated fabrication processes have hindered their practical application.11 The aforementioned excellent graphene properties are suitable for different applications, for example, for flexible organic field effect transistors and, if high transparency is ensured, for solar cells. Indeed, it is well-known that the protection against atmospheric degradation agents still represents a challenge to guarantee acceptable device lifetimes. Chemical vapor deposition (CVD) using a gaseous carbon source, thanks to its capability in producing a controlled area of good quality graphene films, often on copper that gives monolayer graphene,4,12−25 has emerged as the most versatile and promising technique to develop graphene and 2D materials films.26,27 The most common CVD approach, thermal CVD, in hot-wall systems, involves long processing times during which both the whole chamber synthesis temperature and gas © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

December March 20, March 20, March 20,

21, 2017 2018 2018 2018 DOI: 10.1021/acs.iecr.7b05281 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

99.9990 pure argon, and 99.998 pure hydrogen) were mixed to obtain the streams to feed the reactor. For the synthesis, we performed a careful study, starting from an equipment embedded synthesis procedure (procedure no. 1, sample 1, see Supporting Information (SI) Table S1 and Figure S1) and varying the key parameters, such as time, pressure, and feed composition (see Table S1 and S2 for details) to obtain graphene. In particular, sample 8 and sample 9 (see Table S2) were prepared in the best conditions as follows: the copper foil after cleaning in HNO3 was heated to 900 °C for 2 min, under a flow of 120 standard cubic centimeters per minute (sccm) Ar and 10 sccm H2, and successively the temperature was increased to 1000 °C and maintained at this temperature for 900 s to increase copper grain size.4,65,66 During graphene growth, the system pressure regime was set to maintain a chamber pressure of 8 Torr. Subsequently, the CH4 flow was stopped, allowing the system to cool to 100 °C in 630 s, obtaining (i) sample 8 (in the following graphene) keeping a flow of 120 sccm Ar and 10 sccm H2 and (ii) sample 9 keeping a flow of 120 sccm Ar and 10 sccm H2 for the first 300 s and a flow of 10 sccm H2 for the remaining 330 s. At the end, the system was vented, and the samples were taken into air to cool to room temperature. A typical scheme and photographs of the cold wall CVD used for graphene growth on Cu foil are reported in Figure 1a,b. The

that is not performed under ultrahigh vacuum conditions. We found that it is impossible to omit a precleaning step, which we optimize and describe carefully. Moreover, a comprehensive synthesis procedure and an accurate description of how to change the key process parameters (e.g., partial pressure, diffusivity and boundary layer formation), in order to adjust for deviation of composition and temperature, gas purity, etc., are reported. The graphene transfer method, which uses a thermal release tape,4 was adapted for this specific application. In particular, it was designed to obtain a good graphene transfer without altering the PET film (short transfer time, etchant concentration, etc.). PET was chosen as a flexible substrate for its many advantages:43−45 it is one of the most largely used polymers at an industrial scale, it has excellent optical and mechanical properties, it is lightweight, and it has a reasonable cost. The graphene film, synthesized by cold wall CVD, thanks to good continuity and quality works as a transparent barrier on a PET film to be applied with a double function: window electrode and a barrier even for flexible solar cells. PET film after the deposition of only one cold wall CVD prepared graphene layer showed excellent performance as both water and oxygen barrier (e.g., 96% reduction of WVTR (water vapor transmission rate), 95% reduction of OTR (oxygen transmission rate) with respect to bare polymer substrate). These are excellent results also if compared with previously reported papers,46−63 where graphene nanocomposites,49−63 more graphene layers46,47 and one graphene layer but prepared by thermal CVD48 have been previously proposed, thus proving that cold wall graphene CVD is a promising alternative to conventional transparent electrode materials. We speculated also on the possibility to modulate polymer permeability at the nanoscale just applying a graphene layer with a controlled nanoporosity.

2. MATERIALS AND METHODS 2.1. Precleaning Procedure of Copper Foil. A precleaning of the substrate was fundamental to obtain graphene. In particular, two different approaches, using HCl and HNO3 as etching agents, have been explored.64 The substrates were cleaned in HCl using the following procedure: (i) copper foil sonication in acetone for 5 min; (ii) isopropanol washing for 5 min; (iii) drying in flow of nitrogen to remove isopropanol; (iv) immersion in dilute HCl (20 wt %) at 50 °C for 30 s to remove any other surface impurities;64 (v) deionized water washing (3 times); (vi) drying in flow of nitrogen. The first two steps permit efficient removal of all organic contaminants and also any residual of grease or oils coming from the copper foil production process. The substrates were cleaned in HNO3 using the following procedure: (i) copper foil immersion in dilute HNO3 (5 wt %) at room temperature for 30 s, (ii) deionized water washing (3 times), (iii) ultrasonication in acetone for 1 min, (iv) isopropanol washing for 1 min, (v) drying in flow of nitrogen. The two steps in acetone and isopropanol have been performed in shorter times to remove possible residual organic contaminants surviving the more effective HNO3 treatment. 2.2. Graphene Growth Procedure. Graphene was synthesized on copper foils, 10 × 30 mm2 (no. 46365, 25 μm thick, annealed, uncoated, 99.8%, Alfa Aesar), through a commercial cold-wall CVD system from Moorfield (i.e., nanoCVD-8G system). Cylinder gases (99.998 pure methane,

Figure 1. Scheme of cold wall CVD chamber (a). Photographs of the CVD system (b), heating stage (c), and display enlargement (d).

copper foils were loaded on the resistive heating stage (see Figure 1b,c), which is equipped with an embedded thermocouple, mounted on the heater and thus in contact with the substrate.28 In particular, the heater comes out of the chamber for substrate loading, and it is then placed back in for the experiment. The reaction chamber pressure was controlled by pressure control valve. Programmable logic controller electronics equipped with a touchscreen interface (Figure 1d) control the hardware, continuously reporting the heater and chamber temperatures, the pressure, and the gas flow. 2.3. Transfer Procedure of Graphene Films on PET Substrate. The graphene transfer procedure, optimized to obtain a good graphene transfer without altering the PET film, was carried out by the following steps: (i) after synthesis, the bottom of the Cu foil (lying on the heating stage) was polished with nitric acid to remove the graphene grown on the backside; B

DOI: 10.1021/acs.iecr.7b05281 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Atomic force microscopy (Veeco Dimension 3100 with a Nanoscope III Controller) images were obtained to assess the smoothness of the film surface, which is crucial in optoelectronic devices.68

(ii) thermal release tape, supplied from Nitto Denko Co., was attached to graphene film grown on copper; (iii) copper etchant was prepared with a 1 M solution of iron chloride (FeCl3) in water for 3 h; (iv) after etching, the sample was rinsed with deionized water and dried; (v) uniform pressure was applied on a conventional laboratory heating plate at ∼130 °C for 6 s, and then the graphene film was transferred from the thermal release tape to the PET substrate. The polymer substrate thickness used in this study was 23 μm. 2.4. Characterization Methods. Characterization was performed by the combined use of different techniques. SEM pictures were obtained with a LEO 1525 microscope. Raman spectra were obtained at room temperature with a micro-Raman spectrometer, Renishaw inVia, with a 514 nm excitation wavelength. High-resolution transmission electron microscopy (HRTEM) images were acquired using a FEI Tecnai electron microscope operated up to 200 kV with a LaB6 filament as the source of electrons, equipped with an EDX probe. The thermal characterization was carried out with a Mettler differential scanning calorimeter (DSC30), on uncoated and nanocoated samples with the following steps: (i) heating at 20 °C/min from 25 to 300 °C; (ii) isothermal scanning of 5 min at 300 °C; (iii) cooling to 25 °C at 20 °C/min; (iv) heating to 300 °C at 20 °C/min. The heat of melting, ΔHm, and cold crystallization, ΔHcc, were determined by integrating the areas (J/g) under the peaks. The percent crystallinity values Xc were calculated according to the following equation: %Crystallinity = [ΔHm − ΔHcc]/ΔHm0 ×100, where ΔHm is the heat of melting, ΔHcc is the heat of cold crystallization, and ΔHm0 is the heat of melting of the 100% crystalline polymer (135 J/g) taken as reference value.67 X-ray diffraction (XRD) analysis was performed with a Brüker AdvanceD8 diffractometer (Ni-filtered Cu Kα radiation λ = 1.5418 Å, 40 kV, 40 mA). Diffraction patterns were recorded at a scanning rate of 0.5 deg/min. Film-induced frustrated etching (FIFE)48 was performed by dropping, on graphene laying on Cu, a 1 M solution of iron chloride (FeCl3) in water; after waiting few seconds, the etchant was washed by rinsing with deionized water, and then the surface was examined by atomic force microscope (AFM). A broad characterization of the obtained barrier films was performed, including an investigation of the transmittance and the sheet resistance of the graphene/PET. The transparency was evaluated by measuring the UV−visible transmittance of the nanocoated surfaces from 200 to 800 nm with the UV− visible spectrophotometer λ 800, PerkinElmer. All the OTR and WVTR data were averages of triplicate experiments. Conductivity measurements of the graphene/PET system were obtained with a four-point probe surface resistance Keithley 4200 source meter. The OTR (oxygen transmission rate) measurements were carried out with the Permeabilimeter GDP − C 165 provided by Brugger at 23 °C and under oxygen flow of 80 mL/min (according to the standard ISO15165-1); measurements at higher temperature (35 and 45 °C) were also performed for studying a temperature range closer to the actual conditions of photovoltaic (PV) devices. WVTR (water vapor transmission rate) testing was performed with the Permeabilimeter SYSTECH ILLINOIS 7002 at 23 °C and 50% relative humidity.

3. RESULTS AND DISCUSSION 3.1. Cold Wall CVD Process Optimization. Methane flow rate deviations from different equipment, degree of purity of the gases, and different copper foils, together with the intrinsic characteristics of the CVD endothermic processes, are probably the reasons that it is difficult to reproduce results, which are even more crucial in the case of cold wall CVD due to the delicate heat supply to the copper foil surfaces and the shorter synthesis times. Actually there is another key aspect: the pretreatment, which is crucial. Graphene growth is an example of heterogeneous catalysis, involving different steps: diffusion of the precursor through the bulk and the boundary layer to the catalytic surface; adsorption and decomposition to give carbon atoms; transport of carbon on the surface and into the catalyst foil; nucleation and growth of the graphene layer. Graphene formation occurs through nucleation and successive expansion into domains, followed by the union of the domains to form a continuous film. Many parameters can affect the growth of graphene by CVD on a metal surface,69,70 such as pressure, time, hydrocarbon partial pressure, hydrogen partial pressure, inert gas amount, crystallographic orientation, and purity of the foil. In particular, sufficient time and the right carbon precursor partial pressure, because copper is not an inherent limiter of the graphene formation, can lead to a complete coverage of the support with a monolayer. On the other hand, surface purity and control of the crystallographic orientation and of the deleterious sublimation of Cu also are found to be key aspects to obtain good deposition. In order to obtain ordered graphene, we modified the operating conditions, starting from equipment embedded procedure no. 1Table S1, which results in the absence of carbon on the copper surface (Figure S1), according to the following procedures and objectives: • increased synthesis time to cover the copper surface (sample 2 Table S1 and Figure S1); indeed the longer the growth time the more complete the graphene coverage will be;69 • removal of the post-treatment in hydrogen, to avoid the high temperature etching effect69,71,72 (sample 3 Table S1 and Figure S1); • increased pretreatment time to promote the formation of copper grains of suitable size (sample 4 Table S1), until a saturation phenomenon is reached at 1.4 mm, see Figure S2; • modification of the feed composition to increase methane conversion, in particular, reduction of the argon flow rate to increase methane partial pressure and favor a complete coverage (sample 5 Table S1, Figure S1); this is a very delicate aspect, because too high partial pressures of methane induce multilayer nucleation and decreased homogeneity of the sample, and indeed copper is not an inherent limiter of graphene formation;69 • optimization of the low-pressure vacuum conditions (LPCVD) to encourage stream diffusion, control the chemical reaction, and decrease material flow to avoid opposite reactions in the boundary layer70 (sample 6 Table S1, Figure S3); C

DOI: 10.1021/acs.iecr.7b05281 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research • increased synthesis time to promote complete coverage (sample 7 Table S1, Figure S3); • pretreatment optimization; the precleaning due the more delicate heat supply to the copper foil surfaces and the shorter synthesis times was found here to be a fundamental step for homogeneous coating (only a few areas were covered by graphene in the same operating conditions but before cleaning) (sample 8 Table S2, Figure S3); indeed HNO3 etching is very promising because generated NO2 bubbles help to drive impurities away from the copper surface leading to the formation of a continuous and uniform graphene film.64 3.2. Graphene Characterization. Raman spectroscopy is a suitable technique to examine the ordered and disordered crystal structures of carbonaceous materials and the mono-, bi-, and multilayer characteristics of graphene. Figure 2 shows a

typically observed on CVD grown graphene, likely due to the connections between different islands.28 The ID/IG intensity ratio ranges between 0 and 0.6 in the area that contains grain boundaries28 (∼63% of the sample with ID/IG < 0.3, see the histogram in Figure 4a). On the other hand, we do not observe significant D bands, as shown in the histogram of the domain counts measured on five different 40 μm × 40 μm selected areas of Figure S4, when we make measurements on areas that are not affected by misalignments originating from the interface of two different grains. We calculated that more than 99% of the sample is covered by graphene with an ID/IG ratio lower than 0.1 (see Figure 4b). Though Raman analysis can assess the graphene quality through the G and D band intensity ratio, it is arduous to measure defects with high resolution and their distribution on large areas. Film-induced frustrated etching (FIFE), which is an easy method to observe structural defects,48,64 was thus carried out. It is expected that etching of copper occurs in regions that are not covered by graphene, which include partially grown areas, lattice or line defects, and particles on the Cu surface. AFM images of the surface of graphene films on Cu foil after FIFE on sample 8 and sample 9 can be seen in Figure S5. It shows that there are a small number of etch pits, probably due to the particles observed on the graphene surface (see Figure 5b), accounting for less than 5% total area of graphene samples. This good result is probably due to the fact that the sublimation/evaporation of copper leading to the formation of defective areas39,69 is in our case inhibited by the cold walls, which provide much lower temperatures in the environment around the copper foil, and the presence of Ar, because an inert diluent can suppress Cu sublimation.69 SEM images, at different magnification, of the synthesized graphene on Cu foil are shown in Figure 5a−c. It is worth noticing the high continuity of the graphene film, Figure 5a; some wrinkles can be seen but no other significant defects. Only a very small amount of nanoscale particles can be seen on the surface,46 Figure 5b, showing the efficiency of our precleaning procedure for the copper foil. In Figure 5d, a SEM image of sample 9, see also Figure S6, obtained under the same operating conditions of graphene sample 8 but cooled in an etching flow of hydrogen for 330 s, is also reported, showing nanoporosity not covered by carbon. Figure 6 shows the X-ray diffraction patterns of Cu and graphene on Cu. The XRD spectrum of the copper foil (dark cyan line) shows the presence of the only the (100) diffraction peak of copper, as indicated at 2Θ ≈ 50.8°. The XRD spectrum after graphene synthesis on Cu foil (olive line) shows that the support still remained in Cu(100) orientation and the absence of the (002) reflection peak, at about 26° (2Θ), due to graphite interlayer spacing. The growth of graphene by CVD has been found to be dependent on the catalyst surface orientation.69,70 Graphene grown by CVD tends to have polycrystalline structures because of the nucleation and growth of domains with different in-plane orientation and suffers for the presence of grain boundaries, due to the coalescence of graphene domains. Although Cu(111) shows lower mismatch with graphene,80,81 high-quality graphene can grow on the Cu(100) surface,82 which is one of the most used catalytic surfaces.80,83 On the other hand, the use of a catalyst surface with uniform crystallographic orientation and the increase of the Cu grain size24,80,83 are the key factors affecting the quality of our graphene among others.

Figure 2. Typical Raman spectrum of monolayer graphene grown on 99.8% pure 25 μm thick Cu foil (sample 8, Table S2 for growth procedure details), taken on a continuous graphene area in the middle of a copper grain, in the range 1100−3000 cm−1 Raman shift.

typical Raman spectrum of graphene on copper substrate after CVD (sample 8, Table S2 for process condition details) using 514 nm excitation wavelength, focused to a spot size of 5 μm diameter, and ×50 objective lens. The spectrum revealed the most prominent features:70−79 the so-called G band appeared at 1582 cm−1 and the G′ or 2D band at approximately 2700 cm−1.75,76 The 2D band at room temperature exhibits a single Lorentzian feature with a full width at half-maximum (fwhm) of 32 cm−1. The calculated intensity ratio I2D/IG is ∼3, indicating the formation of monolayer graphene.70−79 The D band, due to disorder or edges, at approximately 1350 cm−1, using laser excitation at 2.41 eV, is absent in the spectrum taken on a continuous graphene area grown on a copper grain. Graphene film was transferred on a SiO2/Si substrate and characterized by Raman spectroscopy to check the quality of transferred graphene, too. Typical Raman spectra collected on grain boundaries, in Figure 3, show the three most prominent features, the D band at ∼1350 cm−1, the G band at 1582 cm−1, and the 2D band at approximately 2700 cm−1, of carbonaceous materials.70−79 Full width at half-maximum (fwhm) ranges from 31 to 35 cm−1 indicating CVD grown monolayer graphene. On the continuous films, we observe D peaks indicating defects, D

DOI: 10.1021/acs.iecr.7b05281 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 3. Graphene film optical image after transfer on SiO2/Si, underlying substrate in the upper left corner. Typical Raman spectra of monolayer graphene transferred on SiO2/Si substrate showing the three most prominent features (D band at ∼1350 cm−1, G band at 1582 cm−1, and 2D band at ∼2700 cm−1). Raman maps, measured for an area of 40 μm × 40 μm, containing grain boundaries, using a 50× objective lens, showing fwhm of the 2D peaks, ranging from 31 to 35 cm−1, and ID/IG ratio, ranging from 0 to 0.6.

Figure 4. Histograms of the domain counts for ID/IG in (a) Figure 3 and in (b) Figure S4.

In particular, Table 1 summarizes some of the main results obtained by various kinds of CVD, highlighting that it is possible to obtain graphene through different techniques. The main differences are in the synthesis times, which are shorter in some cases (15 min for rapid thermal CVD, ref 39; 6−12 min for cold wall CVD, ref 28 and this work), and in the vacuum conditions (3.9 × 10−3, ref 69; 1.1 × 10−2, this work). These two aspects strongly affect energy costs, which in our case are

the lowest (see Table 1), with a reduction of up to 99.7% and of 92.6% in comparison with rapid thermal CVD (see Table S3). If the cost of the gases used during the synthesis (it is difficult to evaluate the cost of the pre- and postsynthesis phases) was added, the maximum cost reduction of 98.9% was reached (see Table S5). This is mainly due to the use and cost of Ar, which has a fundamental role in countering the sublimation of E

DOI: 10.1021/acs.iecr.7b05281 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 6. XRD patterns of Cu foil as received and of graphene on Cu foil after synthesis (sample 8 Table S2).

To further evaluate the success of the transfer, SEM and AFM images of the transferred graphene on PET substrate, shown in Figure 8a,b, respectively, are reported. It is possible to observe the high continuity of transferred graphene film and some wrinkles. No defects or holes due to the transfer of graphene are visible on the surface. Furthermore, the transferred graphene is not contaminated with organic adhesive from the thermal release tape. XRD measurements of PET and graphene/PET substrate are shown in Figure 8c. It was found that the XRD profile of the PET substrate was dominant, and it was not possible to obtain any other useful information from this measurement. The data obtained from XRD analysis reveal that there is no change in the PET peak at about 26° (2Θ), after graphene deposition. A further confirmation that graphene deposition does not cause PET modification was given by the results of DSC characterization reported in Table S6. Crystallization modification induced by the thermal treatment for graphene transfer and by the graphene coating were not observed. 3.4. Graphene/PET Properties. The electrical conductivity of the graphene/PET system was measured by a four probe technique. In particular, a sheet resistance4,22,39,48,85,86 of 0.6 kΩ/sq, was obtained. High transparency is necessary to use graphene film as a window electrode for flexible solar cells. In Figure 9a, the transmittance of the polymer sample before and after the graphene coating is shown. Despite some transparency reduction in the wavelength range between 350 and 450 nm, the optical properties was not significantly affected by the graphene deposition (a mean reduction of about 3% can be measured). The final value for the transmittance of about 85%, at a wavelength of 550 nm, may be considered an excellent value for photovoltaic applications. Water vapor transmission rate (WVTR) measurements were also performed. The WVTR value of the PET substrate was as high as 2.84 g m−2 day−1 (see Table 1), which is on the same order of the previously reported values46,48,87 and almost twice the value reported in ref 48. The WVTR value for PET after only one layer graphene deposition decreased significantly to a very low value of 0.11 g m−2 day−1 (see Table 2). This corresponds to about 96% reduction of WVTR compared to bare polymer substrate, which is an excellent result also if compared with previous papers, where 85% and 58%

Figure 5. SEM images of graphene grown on copper foil (sample 8) at different magnifications showing high continuity of graphene film (a, b, c). SEM image of sample 9 obtained in the same operating condition of sample 8 but postsynthesis conditions (d).

copper,69 favoring a high continuity of graphene. When we use ultrahigh vacuum conditions for cold wall CVD in the absence of Ar (CH4 13 sccm; H2 0.8 sccm; synthesis temperature T = 1000 °C; synthesis time 7 min; pretreatment conditions, from room temperature to 900 °C 2 min, from 900 to 1000 °C 15 min in flow of H2), we observe after FIFE a larger number of defects, >15% of total area. As far as the copper foil, the foils vary from one reference to another, but their costs are very similar.28,84 3.3. Graphene on PET. In order to verify the quality of the prepared composite after deposition, graphene on a PET film was characterized using Raman spectroscopy. The Raman spectra of PET and of the transferred CVD graphene on PET substrate are shown in Figure 7. The PET Raman spectrum dominates in all the Raman shift range. On the other hand, the presence of graphene is evidenced by the appearance of the 2D band at approximately 2700 cm−1 in the spectrum of graphene/PET. F

DOI: 10.1021/acs.iecr.7b05281 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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48 69 this work 0.3213 0.0385 0.0154

Graphene supermarket (https://graphene-supermarket.com/Monolayer-Graphene-on-PET-5-pack-Size-1-x1.html).

388 ± 20 Ω/sq on PET 400−800 Ω/sq on SiO2 600 Ω/sq on PET ∼0.16 mainly 50 7.2 ∼2

reductions were reached with 6 graphene layers46 and one thermal CVD graphene layer deposition,48 respectively. WVTR data (Table 2) was also obtained on PET substrates after depositing sample 9 showing nanopores, approximately 5% of its area is not covered by carbon (see Figure S6). In this case, increased WVTR (WVTR = 0.62 g m−2 day−1) and water permeability with respect to PET covered by sample 8 was measured. The water molar flux and permeability values are also reported in Table 2, for PET and PET after sample 3, sample 8, and sample 9 deposition. In particular, permeability is plotted under time increase in Figure 10. When the water vapor concentration is close to zero at the bottom surface, the water vapor permeability can be examined by using the solution88−90 of the Fick’s second law of diffusion: ⎞ ⎛ L2 ⎛ 4L2 ⎞0.5 ∞ Pt = P ⎜ (2n + 1)2 ⎟ ⎟ ∑ exp⎜ − ⎠ ⎝ 4Dt ⎝ πDt ⎠ n = 0

(1)

where P is permeability, with Pt = JtL and P = JL, Jt and J are the water vapor flux at time t and at steady state, L is the sample thickness, and D is the diffusivity. Moreover, P = SD, where S is the solubility (S = c*/p, c* = saturated water concentration, p = partial pressure in the upper side of the film).91 When the pore dimension is considerably larger than those of the water molecules and its thickness is considerably smaller than the mean free path of water molecules, the permeability can be written as Peff = ϕPt + (1 − ϕ)PG

(2)

where Peff, Pt, and PG are the permeability of PET after graphene deposition, PET, and graphene, respectively; ϕ is the surface defect pore density48 (defect pore density area with respect to total area). On the other hand, PG can be considered equal to 0. A slight permeability reduction was observed after sample 3 deposition (see Figure S1); ϕ from eq 2, in this case, is found to be as high as 0.74, while a 1 order of magnitude permeability reduction was detected after sample 8 deposition (see Table 2), ϕ = 0.04, indicating a small amount of defects on the surface probably due to the areas corresponding to the small number of particles observed on the graphene surface (see Figure 5b). This is in good agreement with the result of FIFE analysis showing a small number of pores, size of few hundred nanometers, on the surface of graphene.

a

249 ± 17 Ω/sq on PET 20

13

thermal CVD LP-CVD cold wall CVD LP-CVD rapid thermal CVD LP-CVD LP-CVD cold wall CVD

∼5

30 90 6 360 ∼15

5

Figure 7. Raman spectra of PET and of the transferred CVD graphene (sample 8) on PET substrate.

4.8 × 10−4 3.9 × 10−3 1.1 × 10−2

1030 1000 1000

22 25 28 31 39 0.4191 1.2574 0.0203 5.0294 0.2096 1000 1035 950−1035 1035 970 9.8 2.2 1.3 1.2 7.2 560 Ω/sq on PET

not detectable ∼0.075