Graphene Coatings as Barrier Layers to Prevent the Water-Induced

Oct 5, 2016 - The Weibull distribution is commonly used to analyze the bending test of brittle materials.(37) The modulus of rupture is presented as a...
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Graphene Coatings as Barrier Layers to Prevent the Water-Induced Corrosion of Silicate Glass Bin Wang,† Benjamin V. Cunning,† Sun-Young Park,†,⊥ Ming Huang,†,⊥ Ju-Young Kim,†,⊥ and Rodney S. Ruoff*,†,‡,⊥ †

Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea Department of Chemistry, and ⊥School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea

‡

S Supporting Information *

ABSTRACT: Corrosion-protective coatings for silicate glass based on the transfer of one or two layers of graphene grown on copper by chemical vapor deposition have been demonstrated. The effectiveness of graphene to act as a glass corrosion inhibitor was evaluated by water immersion testing. After 120 days of immersion in water, bare glass samples had a significant increase in surface roughness and defects, which resulted in a marked reduction in fracture strength. In contrast, the single- and double-layer graphene-coated glasses experienced negligible changes in both fracture strength and surface roughness. The anticorrosion mechanism was also studied. KEYWORDS: graphene, glass, coating, barrier, corrosion, mechanical

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he corrosion of silicate glasses by water is a serious problem with implications in many fields including pharmaceutical, industrial, environmental and optical.1−4 The degraded performance of glass, particularly the reduced transmittance of light resulting from surface corrosion, is especially problematic in hot and humid climates.4 Water corrodes silicate glasses through an ionic diffusion and exchange process that results in complex chemical and physical changes near the glass surface.5,6 The chemical mechanism of corrosion is outlined in Figure 1a. The process begins with the adsorption of water on the glass surface. Subsequently, hydrogen ions diffuse into the glass and exchange with alkali metal ions, which then diffuse out of the glass and become solvated. This process aids and occurs in conjunction with the dissolution of the silicate structure.2,5 The relative rates of these chemical processes are dependent on a number of factors including the composition of the glass and pH of the water. In the case of unbuffered water, the initial leeching process results in an increase of pH from accumulation of hydroxide ions in the solution, increasing the corrosion rate. To further examine this process, we measured the pH level of glass immersed in water for a certain number of days as shown in Figure S1. With a fixed ratio of surface area of glass to the volume of water (SA/ V, 12 cm−1), the pH value of the initially neutral water reached above 8 in 3 days at 60 °C and then stabilized at about 8.2 for the following 2 weeks. The constant pH is attributed to the consumption of OH−, forming water by abstraction of a proton from the silanol groups located in glass.7 © 2016 American Chemical Society

Figure 1. (a) Corrosion process of silicate glass by water; (b) graphene coating prevents the erosion of glass by water.

This corrosion process roughens the surface of the glass and, more seriously, reduces its strength. Barbero et al. investigated the effect of a humid environment on the mechanical performance of bundles of glass fibers. 8 After being “conditioned” at high humidity (100%, relative humidity) at Received: July 1, 2016 Accepted: September 29, 2016 Published: October 5, 2016 9794

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Figure 2. (a) Optical micrographs of bare glass and graphene-coated glasses (single-layer graphene coated, G1; two-layer graphene coated, G2). (b) Transparency of graphene-coated glasses (inset, photograph of the different samples). (c) Representative Raman spectra of graphene-coated glasses.

20 °C for 120 days, the conditioned fibers had approximately 75% of their original strength. This reduction in strength was attributed to surface defects on the glass.9,10 To combat this corrosion process, polymeric coatings and carbon nanotubes have been applied to the glass surface to protect it from damage, acting as a diffusion barrier to protect it from reaction with water.11,12 An ideal protective coating should be thin, transparent, and provide a good diffusion barrier to chemical attack. Owing to its excellent chemical barrier properties,13 blocking even He atoms,14 and the recent advances in large-area synthesis15 and subsequent substrate transfer,16 graphene is being explored as a protective layer for materials requiring resistance to corrosion, oxidation, friction, bacterial infection, electromagnetic radiation, etc.17−23 For example, graphene films have been demonstrated to protect copper (Cu), nickel (Ni), and Cu/Ni alloy surfaces from oxidation by both air at elevated temperature of 200 °C and 30% hydrogen peroxide, a liquid at room temperature.24,25 Graphene was also successfully used as a corrosion resistance material to protect metal substrates (Cu, Ni) from electrochemical degradation, which was monitored by electrochemical impedance spectroscopy in an electrolyte.20,26 These findings established graphene as a stable barrier layer to both gas and liquid. Furthermore, as one of the thinnest materials known, the reported optical transmittance for singlelayer graphene is 97.7% in the visible regime,27 making it ideal in applications where transparency is needed. The combination of its low permeability to water,14 chemical inertness,28 and optical transparency make graphene an excellent candidate as a coating material for impeding the water-induced corrosion of glass. In this work, we show that graphene coatings act as excellent inhibitors of water corrosion for silicate glass. In our approach, single-layer graphene films grown by chemical vapor deposition (CVD) were transferred onto glass plates as protective coatings, which were then immersed in deionized water at 60 °C for up

to 120 days (Figure 1b). Corrosion was monitored by changes in the surface features of the samples, and the deleterious effect of this corrosion on the strength of the glass was measured by mechanical failure testing. The anticorrosion mechanism was examined by immersing the samples in deuterium oxide, followed by analysis of the carbon, hydrogen, deuterium, and sodium concentration depth profiles.

RESULTS AND DISCUSSION Graphene Transfer and Characterization. After graphene synthesis, single-layer graphene films were transferred to both sides of a piece of silicate glass via PMMA (poly(methyl methacrylate)) (denoted G1). To examine the advantage of two graphene coatings, we also prepared samples in which we performed the transfer process twice for each side of the glass, to form a graphene bilayer (denoted G2). To minimize both the effect of polymer residue and graphene damage, the PMMA was thermally decomposed after transfer rather than dissolved.29,30 Prior to immersion in water, we surveyed the glass surfaces with optical microscopy and Raman spectroscopy to evaluate the transfer process. The series of images in Figure 2a are optical micrographs of an 80 μm × 80 μm region of each substrate. All micrographs are homogeneous in color, suggesting good conformal contact between the glass and the transferred graphene. G1 was scraped by a tweezer to illustrate the difference between a region of bare glass and one with the graphene coating. The line appearing on the micrograph of G2 is indicative of the graphene film wrinkling during transfer of the second layer.31 The morphology of graphene transferred to a SiOx/Si surface is further shown in Figure S2, where a continuous single-layer film can be observed. The inset in Figure 2b shows the three specimens side by side. The singlelayer and double-layer samples are distinguishable “by eye”, owing to the absorbance of light by graphene on the front and rear of each sample. 9795

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Figure 3. AFM topography images showing surface structural changes of bare and graphene-coated glasses immersed in water. In all images, x (∼10 μm), y (∼10 μm), and z (100 nm) scales are almost identical. For the bare glass samples, two regions of distinct topography appeared after 40 days. Representative topographic images of both these regions are depicted and labeled with their approximate area coverage. The AFM images for all G1 and G2 samples were collected after removal of the graphene coatings by O2 plasma.

The transmittance was measured by detecting the light transmitted through a circular hole with a diameter of 5.3 mm. At a wavelength of 550 nm, the transmittance (normalized to the glass blank) of G1 was measured to be ∼96.0%, an attenuation of approximately twice the absorbance of a single graphene layer, accounting for the coatings on the top and bottom surfaces. In the case of G2, the transmittance was 87.5% (for four layers), an attenuation slightly higher than expected, which we attribute to scattering from the graphene wrinkles observed in the optical micrographs.32 The Raman spectra of the graphene-coated glasses (Figure 2c) show the typical features of graphene: a D band at 1350 cm−1 of very low intensity indicative of a large sp2 carbon crystallite size, a G band at 1586 cm−1, a 2D band at 2687 cm−1 for G1 and 2705 cm−1 for G2. G1 had a 2D/G band intensity ratio of approximately 2, characteristic of single-layer graphene, while G2 had a 2D/G intensity ratio larger than 2, which is associated with a twist angle between the two graphene layers.33 To assess the graphene film over a larger area, we performed Raman mapping of the ID/IG ratio over a 20 μm × 20 μm area (Figure S3a,d). G1 showed an almost uniform ratio over the entire mapped area with defects present in few locations, indicating the good quality of the transferred graphene film. In the case of G2, the mapping showed some regions with a high D band intensity, which we attribute to the graphene wrinkles observed in the optical micrograph (Figure 2a) and also to defects generated during the stacking and transfer processes of these two graphene layers. This also supports the observation that the transmittance measured for G2 was lower than the theoretical value as discussed above. Substrate Immersion and Anticorrosion Performance. After transfer of the graphene to the glass surfaces, the glass plates were immersed in 18 MΩ·cm water, where they were left for either 20, 40, 60, 90, or 120 days at 60 °C before being removed and examined to assess the corrosion impact. Atomic force microscopy (AFM) was used to measure the topography of the samples, allowing direct observation of any corrosion on the glass surfaces. For the samples with graphene coatings, the graphene was removed by oxygen plasma treatment to directly measure the glass surface rather than the graphene surface. A control experiment was done that showed that the oxygen plasma treatment used in removing the

graphene had a negligible effect on the topography of the glass (Figure S4). The results are reported as a series of representative images that were taken to show the surface morphological changes during the exposure to water for the indicated times. The images at each time were selected as being representative after observing several different areas over the sample surface. For bare silicate glass, Figure 3a shows the surface topography progression with time. Characteristically, the surface of the pristine glass before immersion appeared smooth with no apparent pits or peaks on the surface. After 20 days, this surface experienced a dramatic increase in the surface roughness. For bare glass samples analyzed on day 40 and later, two different regions of erosion emerged. One region (denoted A, Figure 3a top) was characterized by a natural progression of the topographic features that appeared after 20 days; however, a second region (denoted B, Figure 3a bottom) also appeared that had a significantly smoother topography. At day 40, the areal coverage of region B as a percentage of the total glass surface was approximately 35%. As immersion time increased, the percentage area of region B grew to dominate the surface of the glass, exceeding 90% after 120 days of immersion, while also developing sharp pits and surface features of its own. This was attributed to physical erosion whereby the sharp surface features were removed from the glass surface, creating a “fresh” surface for further erosion to occur. This phenomenon could be distinguished by the naked eye as seen in Figure S5. The AFM image in Figure S6 shows the boundary between these two regions. These observations suggest the corrosion of glass occurred by simultaneous chemical and physical erosion processes.34 In contrast, glass plates coated with graphene films had a much more stable performance under immersion testing (Figure 3b and c). Besides some rare peaks on the surface, there was limited surface morphological change over 120 days for both G1 and G2. The change with time for both graphenecoated samples showed a slight roughness increase from 0.25 nm to ∼1 nm (Figure S7), thus suggesting that the graphene coating was effective in preventing the surface corrosion of silicate glasses by water, with little difference between the onelayer and two-layer samples. 9796

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ACS Nano Raman mapping of the ID/IG ratio of G1 and G2 was performed after immersion testing to investigate the effect of water on the graphene films. The ID/IG ratio is sensitive to disruption of the sp2 carbon network by both edge defects and chemical modification. In the case of G1 after 60 days’ immersion, small regions appeared on the film with moderate D band intensity (Figure S3). The density and intensity of these regions increased significantly after 120 days, suggesting that some defects were generated gradually on the single-layer graphene coating, and this may be due to oxidation by exposure to water for a long time, as confirmed by X-ray photoelectron spectroscopy (XPS, Figure S8). The C 1s XPS spectra of G1 samples reveal an increased number of oxygen-containing groups, C−O at 286.2 eV and C(O)−O at 289.0 eV, after immersion in water. For G2, the surface already showed significant regions of disorder due to wrinkling. Similarly to G1, the density and intensity of these disordered regions increased as time of immersion increases. An increased number of oxygen-containing groups on the graphene surface was also detected by XPS, Figure S8. It should be noted that we are not able to determine from the Raman map the graphene layer in which these defects occur. An area of the coating may have a highly oxidized outer graphene layer but an intact inner layer still protecting the glass surface. Although graphene was oxidized after immersion in water, we did not observe a significant difference in the graphene layer thickness before and after immersion (Figure S9), suggesting good coverage of the graphene on the glass throughout the length of our experiment. To further elucidate the mechanism by which graphene protects the glass from corrosion, we performed the same immersion experiments using deuterium oxide (D2O) followed by elemental depth analysis of 1H, 2H, 23Na, and 12C using time-of-flight secondary ion mass spectrometry (ToF-SIMS). For bare glass, the diffusion of atmospheric water into the surface created a “hydrated” layer that contains a number of chemical species able to undertake hydrogen exchange.35 These include H2O, H3O+, and silanol groups.36 Accordingly, isotopic analysis of the samples immersed in D2O can give further insight into the role graphene coatings play in providing corrosion protection to glass. To this end, we analyzed the relative concentrations of 1H, 2 H, 23Na, and 12C at various depths using ToF-SIMS for four representative samples: glass stored in air at room temperature, glass immersed in D2O, G1 in D2O, and G2 in D2O at 60 °C for 60 days. The data are presented in Figure 4. The profile of carbon was used to define the onset of glass in the depth profiling. The initial 3 nm depth for G1 and 5 nm for G2 were defined as the coating region, composed of the graphene layers as well as carbon-containing contamination. For the bare glass stored in air, hydrogen was detected down to a depth of ∼40 nm, indicating the thickness of the hydrated layer on the glass before D2O immersion. There was barely any sodium at the surface, and its concentration increased to a maximum at a depth of 20 nm. After immersion in D2O, both hydrogen and deuterium were present at the surface, with the deuterium to hydrogen ratio decreasing with increasing depth, indicating slow diffusion of deuterium into the hydrated layer. Moreover, at the same depth, the sodium concentration of the bare glass sample immersed in D2O was significantly lower than that of bare glass that had been exposed to air. We attribute this removal of sodium near the surface to deuterons, which is consistent with the corrosion mechanism described above.

Figure 4. Elemental profiles measured using the negative ion mode of ToF-SIMS for samples (bare glass, G1, and G2), immersed in D2O at 60 °C for 60 days, and a nonimmersed bare glass sample stored at room temperature in air.

In the case of the glass coated with graphene, deuterium was also present near the glass surface; however the relative abundance was lower than that of the glass−D2O sample, indicating slower diffusion. This observation indicates that deuterium oxide was able to penetrate the graphene to reach the glass surface, albeit at a reduced rate. We ascribe this to both cracks and pinholes formed during the graphene transfer and also the generation of additional defects, which were evident in the Raman spectra after water exposure (Figure S3). Importantly, although deuterium is shown to be able to move through the graphene coating and diffuse into the glass, the subsequent leeching of sodium was attenuated when compared to the bare glass sample immersed in D2O. In the case where there were two graphene layers on the glass, the sodium concentration at a given depth was only slightly lower than that of the original glass. This suggests the graphene coating is inhibiting the exchange of hydrogen ions with sodium ions and not allowing the formation of hydroxide species. Because it is hydroxide ion attack on the siloxane bond that is the primary cause of corrosion, the corrosion is inhibited. Three-point bend tests performed using dynamic mechanical analysis equipment were used to measure the fracture strength of the glasses. The dimensions of each sample were 15 mm × 3.6 mm × 0.15 mm (length × width × thickness). Results were obtained for the original glass, G1, and G2 with water immersion times of 0 (i.e., not immersed), 15, 35, 60, 90, and 120 days. Glass samples were also prepared, but stored in air with a relative humidity of 5% for control experiments. Brittle fracture occurred when the applied load was increased to a critical value and resulted in two or more pieces. The linear stress−strain curves in Figure 5a−d suggest brittle fracture for all samples, which end abruptly, implying quick crack propagation. The similar behavior for all nonimmersed samples indicates that there is no significant effect of the graphene films. The statistical results are summarized and compared in Figure 5e. For glass plates stored in air, the average fracture strength was ∼192 MPa, within experimental error, and did not change over the course of 120 days. In contrast, for the bare glass samples, the average fracture strength decreased gradually over 60 days’ immersion and then remained constant until 120 days. The initial strength decrease was accompanied by the increase of surface roughness, discussed in Figures 3a and S7. The subsequent constant fracture strength is suggested to be due to 9797

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Figure 5. (a−d) Stress−strain responses under loading for different samples, including bare glasses stored in air at room temperature, immersed in water at 60 °C, and graphene-coated glasses in water at 60 °C. (e) Statistical results of the strength for different samples as a function of time. The exact number of specimens measured and analyzed for each set of samples is summarized in Table S1.

Figure 6. (a−d) Probability of fracture for different samples, including bare glasses stored in air at room temperature, glasses immersed in water at 60 °C, and graphene-coated glasses in water at 60 °C. (e) Statistical results of the characteristic strength from Weibull analysis for different samples as a function of time. The exact number of specimens measured and analyzed for each set of samples is summarized in Table S1.

derived characteristic strength and Weibull modulus are compared with the average strength in Table S2. The overlapping strength versus probability of fracture profiles observed for the glass−air samples in Figure 6a indicates a stable strength over 120 days, and similar behavior was also observed for G1 and G2 (Figure 6c,d). In contrast, for samples immersed in water, the profiles for the first 60 days in Figure 6b are different, suggesting a decrease in strength during the initial corrosion process, which later stabilized as shown by the clustering of data points. The characteristic strength of the glass decreased from 200 MPa to 179 MPa after 60 days’ water immersion and stabilized to 179 MPa after 120 days (Figure 6e, Table S2). The characteristic strength for G1 in water changed from 198 MPa to 194 MPa in 120 days, and for G2 from 204 MPa to 198 MPa under the same conditions. This change is similar to the average data analyzed above, although the values

the increasing surface area ratio of regions having comparatively low surface roughness as a result of the erosion mechanism discussed above. For both G1 and G2 samples, the strength remained reasonably constant over the 120 days of immersion. The average strength of G1 decreased from 192 MPa to 188 MPa in 120 days, while that of the G2 changed from 195 MPa to 194 MPa. The changes are negligible considering the uncertainty of the measurements. The Weibull distribution is commonly used to analyze the bending test of brittle materials.37 The modulus of rupture is presented as a characteristic strength (σ0) in this distribution, m ⎡ 1 ⎤ σ ln⎣ P(V ) ⎦ = σ , where P(V) is the survival probability, σ is 0 the applied stress, and m is the Weibull modulus. The measured data for different samples are summarized in Figure 6, and the

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analyzer (Thermo Scientific). ToF-SIMS experiments were performed using a TOF.SIMS 5 from ION-TOF.

are slightly higher. Weibull modulus is a dimensionless parameter used to describe variability in the measured material strength of brittle materials. If the measurements show high variation, the Weibull modulus will be low; otherwise it will be high. The average Weibull moduli for G1 (15) and G2 (18) are larger than for the glass−water samples (11), suggesting that the measurements showed lower variation, and thus, any physical defects were distributed more uniformly throughout the graphene-coated glass samples than the eroded bare glasses. These observations confirm the impressive anticorrosion effect of graphene for silicate glasses.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b04363. pH level of glass immersed solutions for a certain number of days, optical micrograph of a single-layer graphene on a SiOx/Si substrate, Raman maps for G1 and G2 as a function of time in water, AFM comparison of the surface of glass-20days before and after oxygen plasma treatment, typical photos of bare glass plates treated in water for different times, AFM images showing the boundary between the rough and smooth glass surfaces, surface roughness variation of different samples versus time, C 1s XPS spectra for G1 and G2 as a function of time in water, AFM images of graphene coatings for G1 and G2 before and after immersion in water for 120 days, the number of specimens measured for each set of sample, and mechanical properties measured by three-point bend test (PDF)

CONCLUSION This study demonstrates that graphene films are suitable barrier layers to inhibit the water-induced corrosion of silicate glasses. The analysis of the ToF-SIMS experiments shows that the graphene coating inhibits the diffusion of sodium from the glass surface to the water, thus stopping further corrosion caused by the generation of hydroxide species. Although it is at this time challenging to fully cover the glass surface with defect-free graphene films, a single layer of transferred graphene is sufficient to protect the glass from significant surface corrosion and mechanical strength loss. The anticorrosion effect provided by a double-layer graphene coating was marginally better than that provided by a single layer; however a single layer graphene coating may be more promising in practical applications because its transparency is higher. The advent of new largearea graphene synthesis and transfer techniques thus shows great promise for industrial glass coatings.

AUTHOR INFORMATION Corresponding Author

*E-mail: ruoffl[email protected]. Notes

The authors declare no competing financial interest.

METHODS

ACKNOWLEDGMENTS This work was supported by the Institute for Basic Science (IBS-R019-D1). We thank The Sixth Element Materials Technology Co., Ltd., for kindly providing the graphene samples.

Single-layer graphene films were grown on Cu foils by CVD as reported by Li et al.15 The bare glass plates (Corning Inc.; catalog number 2845-18; the size is 1.8 cm × 1.8 cm × 0.15 mm thick) were cleaned by sonication in ethanol and then dried in nitrogen before use. The coating of the glass was conducted using the PMMA-facilitated method, as follows. Briefly, graphene on one side of the Cu foil was first removed by exposing to O2 plasma for 10 min; then PMMA was spin-coated on the other side. After etching away Cu in an aqueous etchant (0.1 M (NH4)2S2O8), the graphene/PMMA film was transferred onto a glass plate and dried in air before transferring one more film onto the other side of the glass. For the double-layer graphene-coated glasses, a stack of two graphene layers was produced by using the graphene/Cu foil to scoop up another layer of graphene/ PMMA film on an aqueous surface as reported previously, and in this way there was no polymer residue left between the two graphene layers.38 The coated glasses were then heated in Ar at ∼400 °C for 2 h to decompose the PMMA.29 A batch of bare glass plates was stored in air with a relative humidity of 5% as control. Glasses and graphene-coated glasses were immersed in unbuffered deionized water for the corrosion experiments. The temperature of the water was ∼60 °C to accelerate the corrosion process. For bend tests, the specimens were cut into a rectangular shape (15 mm × 3.6 mm × 0.15 mm, length × width × thickness) by a dicing saw. A Zeiss optical microscope (AxioCam MRc5) was used to characterize the morphology of the transferred graphene on glasses and SiOx/Si substrates. A Cary Series UV−vis−NIR spectrophotometer (Agilent Technologies) was used to measure the transparency of the glass plates and those coated with graphene films. Raman spectroscopy was performed with a Wi-Tec micro-Raman instrument using 532 nm laser excitation. A Bruker Dimension Icon AFM instrument was used to analyze the surface structural variation of glasses and graphene-coated glasses. Three-point bend tests were carried out using a dynamic mechanical analyzer (DMA Q800, TA Instrument). XPS data were collected using an ESCALAB 250Xi XPS

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DOI: 10.1021/acsnano.6b04363 ACS Nano 2016, 10, 9794−9800