Low-Temperature in Situ Growth of Graphene on ... - ACS Publications

Subscriber access provided by UNIV OF NEBRASKA - LINCOLN

Article

Low-Temperature in Situ Growth of Graphene on Metallic Substrates and Its Application in Anticorrosion Minmin Zhu, Zehui Du, Zongyou Yin, Wenwen Zhou, Zhengdong Liu, Siu Hon Tsang, and Edwin Hang Tong Teo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09453 • Publication Date (Web): 18 Dec 2015 Downloaded from http://pubs.acs.org on December 21, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29

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

ACS Applied Materials & Interfaces

Low-Temperature in Situ Growth of Graphene on Metallic Substrates and Its Application in Anticorrosion Minmin Zhu†‡Δ, Zehui Du‡§, Zongyou Yin‡¶*, Wenwen Zhou‡, Zhengdong Liuǁ, Siu Hon Tsang†§, Edwin Hang Tong Teo†‡§ * †

NOVITAS, School of Electrical and Electronic Engineering and ‡School of Materials Science

and Engineering, Nanyang technological University, 50 Nanyang Avenue, Singapore 639798 ∆

CINTRA CNRS/NTU/THALES and

§

Temasek Laboratories, Nanyang Technological

University, Research Techno Plaza, 50 Nanyang Drive, Singapore 637553 ¶

Department of Materials Science and Engineering, Massachusetts Institute of Technology,

Cambridge, Massachusetts 02139, United States ǁ

Key Laboratory for Organic Electronics & Information Displays and Institute of Advanced

Materials, Nanjing University of Posts &Telecommunications, Nanjing 210046, China * Address correspondence to [email protected] or [email protected]

ABSTRACT Metal or alloy corrosion brings about huge economic cost annually, which is becoming one area of growing concern in various industries, being in bulk state or nano-scale range. Here, single layer or few layers of graphene are deposited on various metallic substrates directly at a low temperature down to 400 oC. These substrates can be varied from hundreds-micrometer bulk metallic or alloy foils to tens of nanometer nanofibers (NFs). Corrosion analysis reveals that both graphene-grown steel sheets and NFs have reduced the corrosion rate of up to ten times lower than that of their bare corresponding counterparts. Moreover, such low-temperature in-situ growth of graphene demonstrates stable and long-lasting anticorrosion after long-term immersion. This new class of graphene coated nanomaterials shows high potentials in anti-corrosion applications for submarines, oil tankers/pipelines and ruggedized electronics.

KEYWORDS: graphene, stainless steel, alloy, nanofiber, electrospinning, anticorrosion 1 ACS Paragon Plus Environment


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

1. INTRODUCTION Metallic corrosion is always a serious concern for many and according to a corrosion cost study by national association of corrosion engineers (NACE), the total cost for corrosion alone in the U.S. exceeds $ 1 trillion annually since 2013.1 Of all, majority of the effort is targeting towards steel, due to its wide range of applications and relatively low cost. To protect various parts and body of the ship from corrosion in a salted and damp environment, its steels, as the basic building blocks, must be either coated with a layer of paint or galvanized on the surface. These paints or galvanized layers usually contain Zn to serve as a sacrificial anode. As Zn is more electronegative than Fe, it is more favorable to form ZnO first and hence preventing the O2 from diffusing into the steels. However, as the Zn depletes (i.e. fully oxidized), the corrosion of the steel will eventually occur. The lifetime of these galvanized layer and paints varies, ranging from a few months to a few years. Recently, graphene, as the strongest and thinnest material that offers a wide range of possibilities in the electrical, optical, and biochemical field,2,3,4 has shown that through the incorporation with paints, it can protect metals from corrosion in brine environment (i.e. an extremely harsh environment) and a few times longer than the normal varnish coating. Besides, wet-transferred monolayer or few-layer graphene films on Cu and Ni foils also demonstrate the excellent capability of anticorrosion and antioxidation. 5 , 6 , 7 , 8 The corrosion rate of graphene coated metal foils were found to be several times slower when compared with their corresponding bare counterparts.5, 9 As graphene is chemically inert, impermeable to gas molecules and stable in ambient atmosphere up to 400 °C, 9,10 it is hence expected that graphene can provide a much more stable and long-lasting anticorrosion protection to steels, as compared with Zn.9,11

2 ACS Paragon Plus Environment

Page 2 of 29

Page 3 of 29

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


However, it is a well-known challenge to synthesis graphene directly on steels since steels have a very low catalytic activity for graphene growth, as compared with Ni and Cu.12,13 Also, the growth of graphene often requires a high growth temperature (i.e. 1000 oC for common CVD process14) that could cause carburization side-effect on the steels during the cooling process.15 Although enhanced chemical vapor deposition (CVD) method, such as microwave or plasma enhanced CVD, can effectively reduce the growth temperature,16,17 still, it can be up to ≥ 850 oC. On the other hand, transferring graphene onto steel is another possible choice, but the size of the graphene is limited and its anticorrosion properties are also degraded significantly after being transferred, mainly because of the adhesion energy between the transferred graphene and the metallic substrates (0.31-0.72 Jm-2), which is ten times lower than that of direct growth of graphene on metals (12.8-72.7 Jm-2).18,19,20 Thus, there is a need to examine a low-temperature in situ growth of graphene, which allows the direct growth of graphene on steels and viable for low-cost, industrial-scale production. In this work, we developed a generalizable low-temperature CVD method to synthesize largearea and low defect graphene on metallic substrates, such as bulk stainless steel sheets and alloy nanofibers (Cu, Ni, CuNi). A multi-heating-zone CVD furnace with solid carbon source (i.e. polystyrene (PS) is used, instead of common gaseous carbon source (CH4 or C2H2). We have successfully grown graphene films on these corresponding substrates at ~ 400 °C. The qualitative and quantitative corrosion test indicates that both of these steel sheets and NFs coated with graphene have the corrosion rate of up to ten times lower than that of bare counterparts. Hence the obtained graphene coated steels have a great potential to replace those commonly-used galvanized steels for applications in corrosive environment, such as submarines, aircraft carriers, oil tankers/pipelines and sewage pipes. Employing our approach, the graphene coated alloy NF



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

(i.e. CuNi) exhibits both the advantages of metallic nanowires and graphene, such as excellent conductivity, antioxidation, and flexibility. All this makes it a promising candidate of a flexible electrodes and interconnection material for various electronic devices, especially those ruggedized electronics.

2. EXPERIMENTAL SECTION 2.1 Metal or alloy nanofiber synthesis by electrospinning. Ni, Cu, and CuNi nanofibers were prepared by the combination of the electrospinning and sol-gel process. In a typical procedure, 0.75 g of Poly(vinylpyrrolidone) (PVP) (Sigma–Aldrich, average molecular weight Mw = 1300000 g/mol) was dissolved in 8.0 ml of methoxy–ethanol (2–MOE) (Sigma–Aldrich, 99.9%), followed by magnetic stirring for 4 h under room temperature to ensure the dissolution of PVP. Subsequently, metal precursors were added to the polymer solution and stirred for 2 h at room temperature to achieve a homogenous composite solution. Metal precursor nickel acetate (NiAc2) and copper acetate (CuAc2) were used to prepare Ni or Cu NFs, respectively. CuAc2 and NiAc2 mixed precursors were used as the metal sources for CuNi NFs synthesis. The precursor solution was delivered to a plastic syringe with a metal needle at the tip. The needle was connected to a high-voltage power supply and positioned vertically on a clamp, with a piece of flat aluminum foil placed 20 cm from the tip of the needle to collect the nanofibers. Upon applying a high voltage of 10 kV, a fluid jet was ejected from the needle tip and polymernanofibers were collected on the aluminum foil or glass at a steady flow rate of 0.5 ml/h. After vacuum drying, calcine, and reduction, metallic NFs were finally obtained. 2.2 Cu nanofiber synthesis by hydrothermal method. 42.5 mg CuCl2·2H2O (the precursor), 34 mg sucrose (the reductant), 240 mg hexadecylamine (HAD, the capping agent), and 25 mL water (the solvent) were mixed and stirred at room temperature overnight for a light blue


Page 4 of 29

Page 5 of 29

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


emulsion obtained. The emulsion was transferred to a Teflon-lined stainless steel autoclave of 40 mL capacity. The autoclave was rotated at 120 °C for 12 h under autogenous pressure and then allowed to cool down to room temperature. The resulting reddish brown solution was centrifuged and then washed with deionized water, n-hexane, and ethanol sequentially. The process was repeated several times to remove the excess surfactant, and a reddish fluffy solid was obtained. The product was kept under n-hexane to avoid the oxidation of the copper NWs. 2.3 Graphene Growth on Different Target Substrates. Although large-area and high quality graphene films have been reported mostly by CVD method using CH4 or C2H2 as a gaseous carbon source,14,21 it is still limited for high growth temperature which is often above 850 oC. Conversely, these metallic nanomaterials used in graphene preparation always possesses low melting point which is usually less than 700 oC. 22 In this case, low-temperature growth of graphene is especially important which can prevent the underlying substrate deform or melt and maintain the formation of graphene at the same time. Bulk substrates (stainless steel, Cu, Ni and CuNi) were first cleaned in acetone, alcohol, deionized-water, and annealed at 700 oC with 150 sccm H2/Ar gas flow for 5 min while the solid carbon source was kept at room temperature. The chamber was then cooled down to the growth temperature, before heating the solid carbon source (i.e. PS, Sigma–Aldrich, Mw = 300 000) to ~ 360 oC for the growth of graphene on targeted substrate. The typical growth time is about 40 min. After growth, the furnace cover was opened and cooled down to room temperature. Using metal or alloy NFs as target substrates, they were annealed in a 200 sccm H2 flow at 700 o

C for 5 min, and then the quartz tube was moved from the furnace. When the temperature of

graphene growth zone was cooled down to the growth temperature, moved the quartz tube into the furnace. Using PS as the solid carbon source, the solid precursor zone temperature was



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

elevated at ~ 360 oC with the H2 and Ar gas flow while maintaining the whole system under low pressure (~ 2 mTorr). After graphene growth process finished, the quartz tube was take out from the furnace and the samples were cooled down to room temperature quickly by natural cooling. Thus, graphene–grown metal or alloy NFs were obtained. 2.4 Sample Characterization. Raman spectroscopy with a laser excitation wavelength of 488 nm was used to characterize the thickness, quality and uniformity of the as-grown graphene samples. The morphology and crystal structures of graphene films on various metal or alloy NFs were characterized by optical microscopy, field-emission scanning electron microscopy (FESEM6340, Oxford), and transmission electron microscopy (TEM, JEOL 2100F, Japan). 2.5 Electrochemical Tests. The quantitative electrochemical corrosion study was carried out in a polytetrafluoroethylene (PTFE) cell. The working electrode (graphene sample) was clamped tightly at the base of the PTFE housing. Ag/AgCl (in 5% seasalt water) electrode and a Pt foil were used as the reference and auxiliary electrodes, respectively. The electrochemical measurements were carried out in an Autolab PGSTAT30 digital potentiostat/galvanostat with FRA2 module.

3. RESULTS AND DISCUSSION In our approach, a multi-heating-zone CVD furnace instead of a common one-heating-zone furnace has been used, which can help to separate the decomposition of solid carbon source and the deposition of carbon on substrates in two different heating zones. This multi-heating-zone CVD system is custom-designed (MTI Corporation, USA), as shown in Figure 1. The solid carbon source in a small ceramic container is placed at the heating zone 1 while the target substrate (steel sheet, NFs, etc.) is placed on the heating zone 3. The heating zone 2 is not used in order to minimize the mutual interference between the heating zone 1 and 3. During the growth


Page 6 of 29

Page 7 of 29

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


process, precursor molecules derived from Zone 1 were taken to Zone 3 by the carrier gases. Then, these carbon source molecules dehydrogenate and form active surface species, which coalesce, nucleate, and assemble the order structure of graphene. The relative growth mechanism will be discussed in the following section. Graphene films were grown on steel sheet (SS 304) using different solid carbon source, such as polystyrene (PS), polyethyl glycerol (PEG), poly (methyl methacrylate) (PMMA) and polyvinylpyrrolidone (PVP). These polymers can be easily dehydrogenated at low temperature and hence produce the active surface species for graphene growth. Figure 2(a) shows the typical Raman spectra of the graphene films derived from different solid carbon sources. There are three very strong peaks located at ∼1347, ∼1570 and ∼2706 cm−1. They are the fingerprints of graphene, namely D, G and 2D peaks. The very high peak intensity of G and 2D peaks indicate that the graphene films are well-crystallized, while the weak D peak indicates the graphene has little inherent defects. Based on the peak intensity of the D band, the graphene films grown with PS carbon source have the lowest defect concentration. It can be seen from Raman spectra in Figure 2(b) that graphene starts to form at temperature as low as ~ 400 oC. The sharp G peaks and obvious 2D peak indicate that graphene film successfully grown on steels. It is well known that the D peak is usually attributed to the presence of defects in graphene. As the temperature increases to 450 oC or above, the crystallinity of graphene significantly improves and the defect concentration decreases at the same time. The morphology and layer number of CVD-grown graphene is strongly affected by H2 which appears to serve a dual role: growth and etching. 23 , 24 Thus, we can control the number of graphene layers grown on steel by tuning the gas flow of H2 and Ar. The two most pronounced peaks in this spectrum are the G peak at 1,580 cm-1 and the 2D peak at 2,690 cm-1. The I2D/IG



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

intensity ratio is about 1.7 and the full-width at half-maximum of the 2D peak is about 37 cm-1, indicating that the graphene is a monolayer,25,26 as shown in Figure 2(c). As the I2D/IG intensity ratio is ≤ 1 or below, it indicates the presence of bilayer or few-layer graphene. It is well-known that there is a significant change in the shape and intensity of the 2D peak when moving from monolayer to few-layer graphene.25 The corresponding 2D peak position of these samples are ~ 2700, 2710, 2716, and 2720 cm-1, respectively. The shifting of the 2D peak position also confirms the change in the layer number of the as-grown graphene. In our approach, a few-layer graphene was obtained when the H2 flow rate was below 8 sccm. With the H2 flow rate increasing to ~15 sccm, bilayer graphene was obtained and when the H2 flow rate was increased to 40 sccm or higher, only monolayer graphene was formed on steel. This is because H2 serves a dual role during the CVD graphene growth: (i) an activator of the surface bound carbon that is necessary for graphene growth and (ii) an etching reagent which can control the morphology and size of the graphene domains, as well as the number of the layers.23,24,27 At a high flow rate, the etching activity is enhanced and the diffusion of active carbon species onto the top layer of graphene is prohibited; as a result, the single-layer graphene growth is favored. When the flow rate reduces, the catalyst surface favors the diffusion of active carbon onto the top layer and hence the growth of bilayer or few-layer is preferred. Besides steel substrate, our approach is also applicable to other bulk metals, such as Cu, Ni and CuNi foil with a slightly higher growth temperature of ~ 450 oC and little defects, as shown in Figure 3. Comparatively, the graphene growing on Ni foil has the least defect. For Ni, our studies shows that the temperature for graphene growth is the lowest among the rest of the metallic substrate (~ 380 oC). As the temperature increases to 450 oC or above, the defect concentration in graphene is significantly reduced (Figure S1 (a)). Similarly, the number of graphene layers


Page 8 of 29

Page 9 of 29

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


grown on the different types of foils can be controlled by tuning the Ar and H2 gas flow. Figure S1 (b) shows the Raman spectra of the graphene films with different layer numbers on Ni foil, as an example. The more efficient graphene growth on Ni foil can be attributed to its high solubility of carbon (2.7%), much higher than the other foils, such as steel (0.12-2%), Cu (0.04%), and CuNi alloy (0.04-2.7%).28,29 Most of reports suggested that the graphene growth on Ni substrate is a typical dissolution precipitation process, while the graphene growth on Cu is a surfacecatalyzed process.30,31,32 The order of the carbon solubility in CuNi and steel is between Ni and Cu, suggesting that the growth mechanism of graphene on them might be complicated. Using the similar low-temperature process, graphene films have also been grown on Ni and CuNi metallic NFs. Polycrystalline Ni and CuNi NFs are fabricated by an electrospinning method.33 Detailed experiment on nanofiber fabrication can be also found in our earlier work.34 Figure 4(a) shows the typical SEM image of Ni NFs with the diameter of about 200 nm. The diameters can be easily changed from hundreds nanometer to a few micrometers by tuning the concentration of Ni precursor in the solution, the solution viscosity and the driving voltage (Figure S2).35 Figure 4(b) and (c) show the low-magnitude and high-magnitude TEM images of graphene directly grown on Ni NFs. The selected area diffraction (SAD) pattern with polycrystalline rings (inset of Figure 4 (c)) shows that the Ni nanofiber is polycrystalline. It can be also seen from TEM result that the low-temperature-grown graphene on Ni NFs is few-layer. As shown in Figure 4(d), three typical Raman peaks at ∼1357, ∼1577 and ∼2718 cm−1 have been investigated, which indicate that graphene has been grown on the Ni nanofiber at low temperature (500 oC). Moreover, lower growth temperature (e.g. 420 oC) is also possible as evidenced by Figure S3. Besides the Ni NFs, we have also successfully grown graphene on CuNi alloy NFs at ~ 450 oC using the similar method (Figure S4). From all the Raman spectrum of the



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

Page 10 of 29

graphene grown at a low temperature, the sharp G peaks and the easily visible 2D peak reveal that we have successfully grown graphene on the polycrystalline metal or alloy NFs. As we known, it is a great challenge to grow graphene on nanofibers with diameter of tens of nanometer, especially for Cu NF which has a much lower melting point below 500 oC when the size of Cu is below 40 nm.36 Figure 5(a) shows the typical SEM image of Cu nanofiber with the diameter of ~ 20 nanometers. Low- and high-magnitude high resolution transmission electron microscopy (HRTEM) images of Cu NFs in Figure 5 (b) and (c) indicate that the Cu nanofiber possesses excellent crystalline structure. The inset SAD pattern in Figure 5 (c) demonstrates the as-grown Cu NF is single crystal. Using these single crystal Cu NFs as the target substrate, graphene can grow at the low temperature of ~ 450 oC, as revealed in Figure 5(d). It is noted that the graphene growth on Cu is a typical surface-catalyzed process due to lower carbon solubility (0.04%); as a result, the monolayer graphene is favored.31 The I2D/IG intensity ratio is ~ 1.8 and the full-width at half-maximum of the 2D peak is about 39 cm-1, indicating the typical features of monolayer graphene.14,25 In summary, we have developed a generalizable, controllable, and cost-effective approach towards the synthesis of large-area and low-defect graphene on metal or alloy substrates at the growth temperature down to 400 oC. The substrates can be varied from hundreds-micrometer bulk metallic or alloy foils to tens or hundreds-nanometer materials, regardless of polycrystalline or single crystal structure. As schematically shown in Figure 6(a) and 6(b), the precursor molecules adsorb on metal or alloy surface and then dehydrogenates or partly dehydrogenate, forming active surface species. Such building blocks energetically active to organize themselves into ordered structure by surface diffusion. It is similar to the process of common CVD process involving carbon atoms.30,37 The decomposition of PS begins at 320 oC and become very fast at


Page 11 of 29

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


390 oC, resulting in a 99% loss in weight at approximately 430 oC.38 So we choose a medium temperature of 360 oC to decompose PS molecule, thereby providing a continuous carbon source in the graphene growth process. It is noted that the dehydrogenateation temperature of PS molecule is much lower than the hexahydric ring decomposition temperature of PS (~ 430 o

C).37,39 On the other hand, a theoretical calculation has been proved that formation energy per

atom of C clusters on Ni surface (~ 0.5-0.7 eV/atom) is lower than that on Cu (~ 0.7-0.9 eV/atom),40 which could explains that the minimum growth temperature on Ni (~ 380 oC) is lower than that on Cu (~ 420 oC). The low-temperature growth of graphene on steel is similar. Corrosion is an electrochemical reaction between a material (metal or alloy) and its environment that produces a deterioration of the material and its properties. At the anode, the metal or alloy are oxidized (corroded) and formed rust or some other corrosion products: Me → Men+ + ne-1 At the cathode, a reduction reaction takes place. This is typically the reduction of oxygen or hydrogen evolution: O2 + 2H2O +4e-1 → 4OH-1 (neutral or alkaline environments) Tafel analysis can help us to measure the corrosion rates of various metals and quantitatively understand the role of graphene in the corrosion reactions between graphene and metallic substrates.7,9 The Butler Volmer equation expresses the exponential dependence of current on the deviation of voltage from the open circuit potential value. So it is possible to extract the reaction kinetic parameter by plotting the current vs. voltage curve. Firstly, we use the graphene coated steel as an example to determine its corrosion rates quantitatively by Tafel analysis. As shown in Figure 7(a), the CV curve of G/Steel is significantly shifted to smaller potentials and lower currents compared to bare steel. A linear fitting is performed on the CV curves by excluding the



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

Page 12 of 29

part of the curve at large over-potentials (~ 0.2 V) and then the Icorr is determined from the intersection point. Therefore, the corrosion rate (R) can be calculated using the Icorr values:41 R corr = (I corr * κ * W equ ) (ρ A )

where the corrosion rate constant κ is 3272 mm/year, the equivalent weight W is 50 g for steel, the density ρ is 8.03 g/cm3, and the sample area A is 2 cm2 for our studies. 42 Here all the corrosion rates presented are obtained by averaging three different samples. As shown in Figure 7(b), the corrosion rate of bare steel sample is 1.75 x 10-13 ± 0.10 x 10-13 (m/s) while that of G/Steel samples is 2.02 x10-14 ± 0.16 x 10-14 (m/s), a reduction of ~ 9 times compared to the bare steel without graphene protection. The value is comparable to that of common CVD-grown graphene on copper at 1000 oC (~7 times).9 Notably, our as-grown G/steel exhibit comparative anti-corrosion ability although the graphene films are grown at much lower temperatures (only 400 - 450 oC). As we know, long-term stability of the anticorrosion coating to the metal substrate is essential for practical applications. Once the graphene coating is damaged or broken, the corrosion rate of the underlying metal or alloy substrates significantly increases.43 Thus, the change in the corrosion rates of G/Steel samples should be investigated after long-term immersion in harsh environment. Figure 7(c) shows the current-voltage curves of G/Steel samples immersed in the sea salt solution as a function of time. The corrosion potentials and corrosion rate are calculated and shown in Figure 7(d). As the beginning, the corrosion potential of G/Steel slightly increases. As the immersion time increases to 30 days, its corrosion rate gradually become a little stable. This indicates that the graphene films can be effective and longlasting anticorrosion coating. The impedance spectra of these samples further prove this point (Figure S6). The impedance of bare steel is linear while that of G/Steel sample seems half round, indicating the graphene coating views as an extra capacitor in the equivalent circuit.9 As the


Page 13 of 29

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


immersion time increases, all the impedance spectra of the samples demonstrate a similar half round shape which suggests that the graphene coating still attaches the steel substrate after a long-term immersion (even 30 days). The anticorrosion capacity of the graphene can also be evidenced by the morphology change of these samples. Figure 7(e) and 7(f) shows the morphology of bare steel and G/Steel before and after electrochemical testing. The graphenecoated steel surface is almost intact, while the one without graphene protection is seriously corroded. Similar phenomenon has also been investigated in graphene-coated nickel foil (Figure S7). As the immersion time gradually changes, the graphene-coated steel has similar smooth surface, with almost no change in its morphology (Figure S8). Furthermore, the corrosion current (Icorr), corrosion potential (Ecorr), and corrosion rate (Rcorr) of all graphene-coated bulk and nanomaterials are listed in Table 1. All graphene-coated target substrates demonstrate a reduction of corrosion rate in seasalt water, which proves again that graphene can be excellent anticorrosion battier for both bulk and nanoscale materials although it is synthesized by multiheating–zone CVD system at a low temperature down to 400 oC. Although it is reported that CVD-grown graphene on Cu is polycrystalline and promotes more extensive wet corrosion than the bare copper due to some grain boundaries, folds, and wrinkles generated by CVD growth or wet transfer process,11 our CVD-graphene grown Ni, steel, and alloy substrates are multilayer which can significantly enhance the impermeability to gases. In addition, graphene multilayer structure may cushion the mechanic stress which build in thick and nonuniform oxide layer and then leads to new avenues for corrosion.

4. CONCLUSION We have developed a generalizable approach to low-temperature in situ growth of graphene on a series target substrates, from hundreds of micrometer thick bulk materials to tens nanometer



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

Page 14 of 29

thin NFs with single crystal or polycrystalline phase. Graphene films with monolayer or a few layers have been successfully grown on a series of bulk and nanomaterial substrates at a low temperature down to 400 oC. Such low temperature growth of graphene is attributed to multiheating-zone CVD system which can separate the decomposition of solid carbon source and the graphene formation on substrates at two different positions. Electrochemical corrosion results show that the low-temperature grown graphene films can significantly slow down the corrosion rate of the substrate materials aforementioned. For example, the corrosion rate of the graphenecoated stainless steel is reduced by 9 times, compared to bare steel. Hence the graphene films exhibit excellent anticorrosion ability and have high potentials in the applications for submarines, oil tankers/pipelines and ruggedized electronics.

ACKNOWLEDGMENTS We thank the financial support of the National Research Foundation (NRF) Proof-of-Concept (POC) grant (No.NRF2011NRF-POC001-048), NTU-A*STAR Silicon Technologies Centre of Excellence under the program grant No. 1123510003 and Singapore Ministry of Education Academic Research Fund Tier 2 No. MOE2013-T2-2-050. We also thank Dr. Ma Bing and Dr. Feng Shuanglong for their support in using the electrochemical measurement facilities.


Page 15 of 29

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


REFERENCES

( 1 ) Jackson, J. Cost of Corrosion Annually in the US Over $1 Trillion, http://www.g2mtlabs.com/2011/06/nace-cost-of-corrosion-study-update/. (2) Zhu, M. M.; Wu, J.; Du, Z. H.; Tay, R.Y. J.; Li, H. L.; Özyilmaz, B.; Teo, E. H. T. A Wafer-Scale Graphene and Ferroelectric Multilayer for Flexible and Fast-Switched Modulation Applications. Nanoscale 2015, 7, 14730-14737. (3) Zeng, S. W.; Sreekanth, K. V.; Shang, J. Z.; Yu, T.; Chen, C. K.; Yin, F.; Baillargeat, D.; Coquet, P.; Ho, H. P.; Kabashin, A. V.; Yong, K. T. Graphene-Gold Metasurface Architectures for Ultrasensitive Plasmonic Biosending. Adv. Mater. 2015, 27, 6163-6169. (4) Wang, J. K.; Xiong, G. M.; Zhu, M. M.; Özyilmaz, B.; Antonio, H. C. N.; Tan, N. S.; Choong, C. Polymer-Enriched 3D Graphene Foams for Biomedical Applications. ACS Appl. Mater. Interfaces 2015, 7, 8275-8283. (5) Chen, S. S.; Brown, L.; Levendorf, M.; Cai, W. W.; Ju, S. Y.; Edgeworth, J.; Li, X. S.; Magnuson, C. W.; Velamakanni, A.; Piner, R. D.; Kang, J. Y.; Park, J. W.; Ruoff, R. S. Oxidation Resistance of Graphene-Coated Cu and Cu/Ni Alloy. ACS Nano 2011, 5, 1321–1327. (6) Kang, D.; Kwon, J. Y.; Cho, H.; Sim, J. H.; Hwang, H. S.; Kim, C. S.; Kim, Y. J.; Ruoff, R. S.; Shin, H. S. Oxidation Resistance of Iron and Copper Foils Coated with Reduced Graphene Oxide Multilayers. ACS Nano 2012, 6, 7763–7769. (7) Nayak, P. K.; Hsu, C. J.; Wang, S. C.; Sung, J. C.; Huang, J. L. Graphene Coated Ni Films: A Protective Coating. Thin Solid Films 2013, 529, 312–316. (8) Krishnamurthy, A.; Gadhamshetty, V.; Mukherjee, R.; Chen, Z.; Ren, W.; Cheng, H. M.; Koratkar, N. Passivation of Microbial Corrosion Using a Graphene Coating. Carbon 2013, 56, 45–49. ( 9 ) Prasai, D.; Tuberquia, J. C.; Harl, R. R.; Jennings, G. K.; Bolotin, K. I. Graphene: Corrosion-Inhibiting Coating. ACS Nano 2012, 6, 1102–1108. 15 ACS Paragon Plus Environment


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

Page 16 of 29

(10) Bunch, J. S.; Verbridge, S. S.; Alden, J. S.; Van der Zande, A. M.; Parpia, J. M.; Craighead, H. G.; McEuen, P. L. Impermeable Atomic Membranes from Graphene Sheets. Nano Lett. 2008, 8, 2458–2462. (11) Schriver, M.; Regan, W.; Gannett, W. J.; Zaniewski, A. M.; Crommie, M. F.; Zettl, A. Graphene as a Long-Term Metal Oxidation Barrier: Worse Than Nothing. ACS Nano 2013, 7, 5763-5768. (12) Tan, L. F.; Zeng, M. Q.; Zhang, T.; Fu, L. Design of Catalytic Substrates for Uniform Graphene Films: From Solid-Metal to Liquid-Metal. Nanoscale 2015, 7, 9105-9121. (13) Seah, C. M.; Chai, S. P.; Mohamed, A. R. Mechanisms of Graphene Growth by Chemical Vapour Deposition on Transition Metals. Carbon 2014, 70, 1-21. (14) Li, X. S.; Cai, W. W.; An, J.; Kim, S.; Nah, J.; Yang, D. X.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Large-Area Synthesis of HighQuality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312-1314. (15) Gullapalli, H.; Reddy, Arava L. M.; Kilpatrick, S.; Dubey, M.; Ajayan, P. M. Graphene Growth via Carburization of Stainless Steel and Application in Energy Storage. Small 2012, 7, 1697-1700. (16) John, R.; Ashokreddy, A.; Vijayan, C.; Pradeep, T. Single- and Few-layer Graphene Growth on Stainless Steel Substrates by Direct Thermal Chemical Vapor Deposition. Nanotechnology 2011, 22, 165701. (17) Yuan, G. D.; Zhang, W. J.; Yang, Y.; Tang, Y. B.; Li, Y. Q.; Wang, J. X.; Meng, X. M.; He, Z. B.; Wu, C. M. L.; Bello, I.; Lee, C. S.; Lee, S. T. Graphene Sheets via Microwave Chemical Vapor Deposition. Chem. Phys. Lett. 2009, 467, 361-363. (18) Steven, P. K.; Narasimha, G. B.; Martin, L. D.; Bunch, J. S. Ultrastrong Adhesion of Graphene Membranes. Nat. Nanotechnol. 2011, 6, 543-546. (19) Yoon, T.; Shin, W. C.; Kim, T. Y.; Mun, J. H.; Kim, T. S.; Cho, B. J. Direct Measurement of Adhesion Energy of Monolayer Graphene As-Grown on Copper and Its Application to Renewable Transfer Process. Nano Lett. 2012, 12, 1448-1452. (20 ) Santanu, D.; Debrupa, L.; Lee, D. Y.; Arvind, A.; Choi, W., Measurements of the Adhesion Energy of Graphene to Metallic Substrates. Carbon 2013, 59, 121-129.


Page 17 of 29

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


(21) Qi, M.; Ren, Z. Y.; Jiao, Y.; Zhou, Y. X.; Xu, X. L.; Li, W. L.; Li, J. Y.; Zheng, X. L.; Bai, J. T. Hydrogen Kinetics on Scalable Graphene Growth by Atmospheric Pressure Chemical Vapor Deposition with Acetylene. J. Phys. Chem. C 2013, 117, 14348-14353. (22) Su, X.; Zhang, Z. J.; Zhu, M. M. Melting and Optical Properties of ZnO Nanorods. Appl. Phys. Lett. 2006, 88, 061913. (23) Vlassiouk, I.; Regmi, M.; Fulvio, P.; Dai, S.; Datskos, P.; Eres, G.; Smirnov, S. Role of Hydrogen in Chemical Vapor Deposition Growth of Large Single-Crystal Graphene. ACS Nano 2011, 5, 6069-6076. (24) Sun, Z. Z.; Yan, Z.; Yao, J.; Beitler, E.; Zhu, Y.; Tour, J. M. Growth of Graphene from Solid Carbon Sources. Nature 2010, 468, 549-552. (25) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 2006, 97, 187401-4. (26) Ferrari, A. C; Basko, D. M. Raman Spectroscopy As a Versatile Tool for Studying the Properties of Graphene. Nat. Nanotechnol. 2013, 8, 235-246. (27) Zhang, X. Y.; Wang, L.; Xin, J.; Yakobson, B. I.; Ding, F. Role of Hydrogen in Graphene Chemical Vapor Deposition Growth on a Copper Surface. J Am. Chem. Soc. 2014, 136, 20403047. (28) Dai, B. Y.; Fu, L.; Zhou, Z. Y.; Xu, H. T.; Wang, S.; Liu, Z. F. Rational Design of a Bilayer Metal Alloy for Chemical Vapour Deposition Growth of Uniform Single-layer Graphene. Nat. Commun. 2011,522, 1–6. (29) Liu, N.; Fu, L; Dai, B. Y.; Yan, K.; Liu, X.; Zhao, R. Q.; Zhang, Y. F.; Liu, Z. F. Universal Segregation Growth Approach to Wafer-size Graphene From Non-noble Metals. Nano Lett. 2011, 11, 297-303. (30) Li, X. S.; Cai, W. W.; Colombo, L.; Ruoff, R. S. Evolution of Graphene Growth on Ni and Cu by Carbon Isotope Labeling. Nano Lett. 2009, 9, 4268-4272. (31) Losurdo, M.; Giangregorio, M. M.; Capezzuto, P.; Bruno, G. Graphene CVD Growth on Copper and Nickel: Role of Hydrogen in Kinetics and Structure. Phys. Chem. Chem. Phys. 2011, 13, 20836–20843.



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

Page 18 of 29

(32) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J. H.; Kim, P.; Choi, J. Y.; Hong, B. H. Large-scale Pattern Growth of Graphene Films for Stretchable Transparent Electrodes. Nature 2009,457, 706–710. (33) Sun, Z.; Zussman, E.; Yarin, A. X.; Wendorff, J. H.; Greiner, A. Compound Core-shell Polymer Nanofibers by Co-electrospinning. Adv. Mater. 2003, 22, 1929-1932. (34) Liu, Z. D.; Yin, Z. Y.; Du, Z. H.; Yang, Y.; Zhu, M. M.; Xie, L. H.; Huang, W. Low Temperature Growth of Graphene on Cu–Ni Alloy Nanofibers for Stable, Flexible Electrodes. Nanoscale 2014, 6, 5110-5115. ( 35 ) Teo, W. E.; Ramakrishna, S. A Review on Electrospinning Design and Nanofibre Assemblies. Nanotechnology 2006, 17, 89-106. (36) Yeshchenko,O. A.; Dmitruk, I. M.; Alexeenko, A. A.; Dmytruk, A. M. Size-dependent Melting of Spherical Copper Nanoparticles Embedded in a Silica Matrix. Phys. Rev. B 2007, 75, 085434-6. (37) Xue, Y. Z.; Wu, B.; Jiang, L.; Guo, Y. L.; Huang, L. P.; Chen, J. Y.; Tan, J. H.; Geng, B. L.; Hu, W. P.; Yu, G.; Liu, Y. Q. Low Temperature Growth of Highly Nitrogen-Doped Single Crystal Graphene Arrays by Chemical Vapor Deposition. J. Am. Chem. Soc. 2012, 134, 11060– 11063. (38) David, A. A.; Eli, S. F. The Kinetics of the Thermal Degradation of Polystyrene and Polyethylene. J. Polym Sci. 1961, 54, 253-260. (39) Johns, I. B.; McElhill, E. A.; Smith, J. O. Thermal Stability of Some Organic Compounds. J. Chem. Eng. Data 1962, 7, 277-281. (40) Yuan, Q. H.; Gao, J. F.; Shu, H. B.; Zhao, J. J; Chen, X. H.; Ding, F. Majic Carbon Clusters in the Chemical Vapor Deposition Growth of Graphene. J. Am. Chem. Soc. 2012, 134, 2970–2975. (41) GAMRY redefining electrochemical measurement: Basics of electrochemical corrosion measurements.

http://www.gamry.com/application-notes/basics-of-electrochemical-corrosion-

measurements/. (42) Loto, C. A.; Popoola, A. P. I.; Fayomi, O. S.; Loto, R. T. Corrosion Polarization Behavior of Type 316 Stainless Steel in Strong Acids and Acid Chlorides. Int. J. Electrochem. Sci. 2012, 7, 3787-3797.


Page 19 of 29

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


(43) Ambrosi, A.; Pumera, M., The Structural Stability of Graphene Anticorrosion Coating Materials is Compromised at Low Potentials. Chem. Eur. J. 2015, 21, 7896 –790.

FIGURE CAPTIONS: Figure 1. Schematic of the CVD growth for solid carbon source on the target substrates. The solid carbon source (i.e. PS) is placed in heating zone 1 while the target substrate is hold on the heating zone 3. There are two controller system for heating zone 1 and 3, respectively. Figure 2. Raman spectra of (a) graphene films grown on stainless steel at 500 oC using PS, PVP, PEG, and PMMA as solid carbon sources, and (b) Raman spectra of graphene grown on stainless steel at 600, 550, 500, 450, and 400 oC. (c) Raman spectra of the PS-derived graphene samples with controllable layer numbers at 500 oC by varying the gas flow rate. Figure 3. Raman spectra of PS-derived graphene grown on bulk Ni, Cu, CuNi, and stainless steel target substrates at 500 oC. Figure 4. (a) SEM image of Ni nanofibers by electrospinning. (b) Low-magnitude and (c) highmagnitude HRTEM images of graphene grown directly on the Ni nanofibers. The inset of (c) is the SAD of the graphene/Ni nanofiber. (d) Raman spectrum of the G/Ni nanofiber. Figure 5. (a) SEM image of Cu nanofibers by hydrothermal method. (b) Low-magnitude and high-magnitude HRTEM images of the crystal Cu nanofibers. The inset of (c) is the SAD of the Cu nanofiber. (d) Raman spectrum of the graphene grown directly on the crystal Cu nanofiber. Figure 6. (a) Schematic diagram of the graphene growth on a metal or alloy surfaces with polystyrene as carbon source by LPCVD. (b) Formation of C-H and C-C bonds due to decomposition of PS.



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

Page 20 of 29

Figure 7. (a) Tafel plots of steel and G/steel samples in 5% sea salt solution. (b) Corrosion rates of stainless steel and G/steel samples extracted from Tafel plots. (c) The dependence of the current-voltage curve vs time and (d) the corrosion potentials and corrosion rate as a function of the time in the G/Steel sample. Optical images of (e) pure stainless steel and (f) G/Steel substrate before and after electrochemical anticorrosion testing.

Figure 1. Schematic of the CVD growth for solid carbon source on the target substrates. The solid carbon source (i.e. PS) is placed in heating zone 1 while the target substrate is hold on the heating zone 3. There are two controller system for heating zone 1 and 3, respectively.


Page 21 of 29

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




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

Page 22 of 29

Figure 2. Raman spectra of (a) graphene films grown on stainless steel at 500 oC using PS, PVP, PEG, and PMMA as solid carbon sources, and (b) Raman spectra of graphene grown on stainless steel at 600, 550, 500, 450, and 400 oC. (c) Raman spectra of the PS-derived graphene samples with controllable layer numbers at 500 oC by varying the gas flow rate.


Page 23 of 29

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


Figure 3. Raman spectra of PS-derived graphene grown on bulk Ni, Cu, CuNi, and stainless steel target substrates at 500 oC.



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

Page 24 of 29

Figure 4. (a) SEM image of Ni nanofibers by electrospinning. (b) Low-magnitude and (c) highmagnitude HRTEM images of graphene grown directly on the Ni nanofibers. The inset of (c) is the SAD of the graphene/Ni nanofiber. (d) Raman spectrum of the G/Ni nanofiber.


Page 25 of 29

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


Figure 5. (a) SEM image of Cu nanofibers by hydrothermal method. (b) Low-magnitude and high-magnitude HRTEM images of the crystal Cu nanofibers. The inset of (c) is the SAD of the Cu nanofiber. (d) Raman spectrum of the graphene grown directly on the crystal Cu nanofiber.



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

Page 26 of 29

Figure 6. (a) Schematic diagram of the graphene growth on a metal or alloy surfaces with polystyrene as carbon source by LPCVD. (b) Formation of C-H and C-C bonds due to decomposition of PS.


Page 27 of 29

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


Figure 7. (a) Tafel plots of steel and G/steel samples in 5% sea salt solution. (b) Corrosion rates of stainless steel and G/steel samples extracted from Tafel plots. (c) The dependence of the current-voltage curve vs time and (d) the corrosion potentials and corrosion rate as a function of the time in the G/Steel sample. Optical images of (e) pure stainless steel and (f) G/Steel substrate before and after electrochemical anticorrosion testing.



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

Page 28 of 29

Table 1. By extrapolating the Tafel anodic and cathodic linear parts until they intersect as straight lines, the corrosion current (Icorr), corrosion potential (Ecorr), corrosion rate (Rcorr), and corrosion reduction times can be deduced, respectively. Icorr (A/cm2)

Ecorr (V)

Rcorr (m/s)

Steel

5.41E-5

0.188

1.75E-13

G/Steel

6.25E-5

0.167

2.02E-14

Cu

5.61E-5

0.210

3.75E-13

G/Cu

6.64E-5

0.176

2.42E-14

Ni

5.78E-6

0.215

6.65E-14

G/Ni

6.95E-7

0.191

3.22E-15

Ni NF

5.88E-6

0.211

6.22E-14

G/Ni NF

6.86E-7

0.187

3.35E-15

CuNi NF

5.01E-5

0.193

4.85E-13

G/CuNi NF

6.15E-5

0.179

2.92E-14

Cu NF

5.11E-5

0.185

4.15E-13

G/Cu NF

6.35E-5

0.173

2.98E-14


Reduction times

8.6

15.4

20.6

18.6

16.6

13.9

Page 29 of 29


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

ACS Paragon Plus Environment

Low-Temperature in Situ Growth of Graphene on ... - ACS Publications

Recommend Documents