Comprehensive electrochemical study on corrosion

Corrosion Science 130 (2018) 31–44

Contents lists available at ScienceDirect

Corrosion Science journal homepage: www.elsevier.com/locate/corsci

Comprehensive electrochemical study on corrosion performance of graphene coatings deposited by chemical vapour deposition at atmospheric pressure on platinum-coated molybdenum foil ⁎⁎

Samira Naghdia, Katarina Nešovićb, Vesna Mišković-Stankovića,b, , Kyong Yop Rheea, a b

MARK

⁎

Department of Mechanical Engineering, Kyung Hee University, 17104 Yongin, Republic of Korea Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000 Belgrade, Serbia

A R T I C L E I N F O

A B S T R A C T

Keywords: B. Raman spectroscopy B. XPS B. EIS B. Cyclic voltammetry B. Polarization

In this work, we present the results of electrochemical, Raman, FE-SEM/EDS, and XPS studies of graphene coatings deposited by chemical vapour deposition at atmospheric pressure on Mo foil and Pt-coated Mo foil as catalysts. The results showed that the graphene coating synthesized on Pt-coated molybdenum foil was bi-layer and had fewer defects, while the graphene coating on Mo foil was few-layer with more defects. The corrosion studies indicated that both graphene coatings on bare Mo and Pt-coated Mo exhibited good protective properties and can act as a barrier against corrosion in 0.1 M NaCl.

1. Introduction Corrosion and corrosion protection are serious concerns for many industries. Mo is a transition metal with versatile applications as it is increasingly used in the electrical industry; for example, in supercapacitors and electronic and photonic devices [1–4] and as back contacts for solar cells and catalysts [5,6]. Although Mo generally has good corrosion resistance in mild environments, there is a need for development of thin-layer coatings for protection of Mo in order to increase and prolong its stability when exposed to corrosive media. Several studies were devoted to the investigation of the Mo corrosion mechanism [7,8]. Generally, Mo is less susceptible to corrosion in acidic environments than in neutral or alkaline conditions owing to the existence of passivation regions in acidic media [7,9,10]. On the contrary, there was no observation of passive film formation in alkaline electrolytes [7,11]. The mechanism of anodic oxidation of Mo is complicated due to the formation of several different species at anodic overpotentials depending on the electrolyte pH. The surface of Mo is always more or less covered by a layer of mixed oxide, consisting mainly of Mo (III) and Mo(IV) species, that even forms spontaneously at open circuit potentials [12,13] and undergoes changes in several charge transfer steps after polarization in solutions of different pH [8]. In acidic environments, a MoO2 oxide layer is formed first and then is converted through a series of electron transfer steps to MoO3, which is believed to be the main cause of passivation of Mo (with the mixed oxide-hydroxide

⁎

also containing MoO2 and MoO(OH)2), observed as a passive region in anodic polarization curves [14]. According to the Pourbaix diagram of Mo [15], MoO2 is quite unstable at higher anodic overpotentials in neutral and alkaline solutions and is subjected to dissolution through the formation of intermediate Mo(III) and Mo(V) species, with the final soluble products being Mo(VI) species such as HMoO4− and MoO42− [7,16,17]. Many different methods of corrosion prevention have been developed, but researchers are currently paying attention to the new and robust coatings that improve electrical and thermal properties and appearance of the protected metals [18]. The introduction of the versatile material graphene has prompted many changes in diverse applications [19]. Since the discovery of graphene in 2004, this monolayer of carbon atoms, arranged in a honeycomb lattice, has emerged as a material that exhibits unparalleled electrical, thermal, and mechanical properties [20]. The vast application of graphene relies on the attractive properties of this two-dimensional material such as high carrier mobility, large theoretical specific area, good electrical and thermal conductivity, high Young’s modulus, great optical transmittance, and chemically inert nature [19,21–23]. The ability of graphene to prevent ionic transport has rendered graphene a prospective protection barrier for metallic substrates against corrosion [24,25]. In recent years, there have been an increasing number of studies on graphene as thin-layer anti-corrosion coatings on various metal substrates, such as Cu, Co, Ni, and Fe [26–28], as well as filler for enhanced protective properties of

Corresponding author. Corresponding author at: Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000 Belgrade, Serbia. E-mail addresses: [email protected] (V. Mišković-Stanković), [email protected] (K.Y. Rhee).

⁎⁎

http://dx.doi.org/10.1016/j.corsci.2017.10.021 Received 5 June 2017; Received in revised form 23 October 2017; Accepted 25 October 2017 Available online 29 October 2017 0010-938X/ © 2017 Elsevier Ltd. All rights reserved.


S. Naghdi et al.

Fig. 1. (a) Pre-treatment of Mo foil, (b) CVD of graphene on Mo (Mo-G), and (c) CVD of graphene on Pt-coated Mo (Mo/ Pt-G).

dependent on the Pt grain size, which was changed by pre-annealing the catalyst. Sun et al. demonstrated the dependence of high-quality multilayer graphene synthesis on proper control over the growth kinetics when using Pt as catalyst [39]. In our previous research, we successfully synthesised CVD-grown graphene coatings with good barrier properties on Cu and Al [28] and, most recently, on Mo [24]. In this work, we present graphene coatings deposited by chemical vapour deposition at atmospheric pressure on Ptcoated Mo foils. Building on our previous work, we compare the catalytic properties of bare Mo and Mo with Pt coating for the growth of uniform graphene layers and investigate the effect of Pt thin film on Mo surface on the quality of graphene and its corrosion protection properties.

polymer coatings [29–31]. Many fabrication techniques including mechanical exfoliation of graphite, epitaxial growth on SiC, reduction of graphene oxide, unzipping of carbon nanotubes (CNTs), and chemical vapour deposition (CVD) have been investigated to produce highquality graphene with good uniformity and size. Among these methods, thermal CVD, as the most scalable and inexpensive deposition technique, has emerged as a common approach for single and few-layer graphene synthesis on metal substrates [32,33]. However, CVD-grown graphene coatings can have many defects and small domain sizes, which can lead to enhanced corrosion of the metal due to intercalation of electrolyte along the grain boundary [34,35]. For this reason, various types of metal catalysts have been carefully chosen to mediate the mechanism of graphene growth [36], which is directly dependent on the distinctive solubility of carbon in different catalysts [37]. Pt is one of the known catalysts that, unlike the widely used catalysts (Cu and Ni), is more resistant to oxidation due to its inertness [38] and has better catalytic ability for decomposition of hydrocarbons and subsequent graphitization than Cu [39]. Unlike Cu, which requires a lowpressure environment to grow graphene, Pt can induce growth of largegrain graphene at ambient pressure [40], while the weak interaction of Pt substrate with graphene helps reduce the negative effect of the substrate on the quality of graphene [41,42]. Yong et al. reported the growth of uniform bi-layer graphene with giant grain size on Pt thin film [38]. They showed that the sizes of graphene grains were

2. Materials The reaction gases of hydrogen (H2, 99.999%), argon (Ar, 99.999%), and methane (CH4, with 99.995% purity) were purchased from A-Rang Gas, Korea. A 0.25-mm-thick, annealed, uncoated 99.95% pure Mo foil was purchased from Alfa Aeser, USA. Automated thermal CVD equipment, from Scientific Engineers (S. Korea), was utilized to deposit graphene on the Mo foils. NaCl (Sigma Aldrich, USA) and deionised water from the Milli-Q system (Millipore, USA) were used in all electrochemical experiments. 32


S. Naghdi et al.

Fig. 2. FE-SEM images of (a, b) bare Mo foil, (c, d) Mo/Pt (Pt-coated Mo foil), (e, f) Mo-G (graphenecoated Mo foil), (g, h) Mo/Pt-G (graphene-coated Mo/Pt), in two different scales, before immersion in 0.1 M NaCl.

3. Experimental

rate) using the same starting gas mixture employed in the heating step. After the pre-treatment step, samples were withdrawn from the furnace, and some of the samples coated with Pt thin film (100 nm thickness) using an automatic Quorum Tech Sputter Coater (model Q150RS, pressure 7 × 10−3 mbar, deposition time 220 s, Pt solid disc as a source). Pt-coated molybdenum foil was referred to as Mo/Pt. For graphene deposition, the Mo foils and Pt-coated Mo foils were placed into a quartz tube furnace with a gas flow of 200 sccm Ar and 200 sccm H2 at atmospheric pressure while the temperature was increased to 1000 °C. After 20 min at this temperature, the samples were exposed to the CH4/H2 gas mixture (100 and 200 sccm, respectively) for 5 min, and then the furnace was cooled to room temperature (100 °C/min cooling rate) using the same starting gas mixture employed in the heating step. Here, obtained samples are named Mo-G (graphene coating on Mo foil) and Mo/Pt-G (graphene coating on Pt-coated Mo foil). Fig. 1 shows the different steps of sample preparation.

Graphene was grown by chemical vapour deposition at atmospheric pressure (APCVD) on Mo foils, with CH4 serving as the carbon precursor. In previous works [24,25,32], we found the best pre-treatment conditions for Mo substrate. Namely, Mo foils were first washed ultrasonically in acetone (15 min), followed by washing in ethanol (15 min) and immersion in 1 M nitric acid for 15 min at room temperature. The Mo foils were then placed into a quartz tube furnace with a gas flow of 200 sccm (standard cubic centimetres per minute) Ar and 200 sccm H2 at atmospheric pressure while the temperature was increased to 1000 °C. Samples were annealed at 1000 °C for 15 min to remove any impurities and metal oxide from the surface. At this temperature, the remnant native metal oxide was removed, followed by recrystallization of the Mo substrate. The growth of the Mo grains results in higher probability of obtaining a larger graphene domain size. Then the furnace was cooled to room temperature (100 °C/min cooling 33


S. Naghdi et al.

Fig. 3. FE-SEM images of (a) bare Mo, (b) Mo/Pt (Ptcoated Mo foil), (c) Mo-G (graphene-coated Mo foil) and (d) Mo/Pt-G (graphene-coated Mo/Pt) after 30 days immersion in 0.1 M NaCl.

4. Characterization

obtained.

The quality of the obtained graphene coatings on Mo and Mo/Pt foils was investigated using the Raman spectrophotometer RFS 100/S, λ = 532 nm (Bruker, USA). Field-emission scanning electron microphotographs (FE–SEM) were obtained using LEO SUPRA 55 (Carl Zeiss AG, Germany), equipped with an energy dispersive X-ray spectroscopy (EDS) instrument EDAX GENESIS 2000 (EDAX Inc., USA). X-ray photoelectron spectroscopy (XPS) was performed using a K-Alpha System (Thermo Electron, USA) spectrometer equipped with an Al Kα monochromatic X-ray source (1486.7 eV) and a micro-focused monochromator. Electrochemical experiments were carried out in a three-electrode Teflon electrochemical cell. The samples of molybdenum foils (bare Mo, Mo/Pt, Mo-G, and Mo/Pt-G) served as working electrodes with a 1 cm2 surface area, obtained by placing rubber ring between the electrode and cell wall. The reference electrode (saturated calomel electrode, SCE) was connected externally via a Luggin capillary tube, and a platinum mesh electrode served as the counter. All potentials in this paper are reported vs. SCE. Potentiodynamic sweep (PDS), linear polarization (LPR), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) measurements were all performed on a Reference 600™ Potentiostat/Galvanostat/ZRA (Gamry Instruments Inc., USA). All scans were performed on the samples after different times of immersion (30 min to 30 days) in 0.1 M NaCl (aq.) electrolyte. Before each corrosion measurement, open circuit potential (Eocp) was monitored until at least 0.01 mV/10 s stability was obtained. For the PDS measurements, the sample was polarized in the potential range of −150 mV to +150 mV with respect to open circuit potential (Eocp), starting from the cathodic region, with a scan rate of 0.5 mV/s. Linear polarization of the samples was carried out in the potential range of −5 mV to +5 mV with respect to Eocp at a scan rate of 0.125 mV/s in order to calculate the values of the polarization resistance. The impedance measurements were carried out at the open circuit potential by applying sinusoidal voltage variation of 10 mV amplitude around Eocp in the frequency range of 100 kHz–10 mHz. The fitting and analysis of the EIS spectra were performed using Echem Analyst software (Gamry Instruments, Inc., USA). Cyclic voltammograms were recorded in the potential range from −1.0 V to 0.2 V vs. SCE, starting from Eocp (scan rate 20 mV/s). Each experiment was continued until stationary voltammograms were

5. Results and discussion 5.1. FE-SEM and energy dispersive X-ray spectroscopy (EDS) Fig. 2a–h shows FE-SEM images of bare Mo, Mo/Pt, Mo-G, and Mo/ Pt-G foils before exposure to 0.1 M NaCl. Since graphene is highly transparent and thin, the SEM images can only display the surface morphology of the Mo foil beneath graphene, for which there are noticeable morphological differences between investigated samples. As is obvious in Fig. 2a and b, the surface of bare Mo foil is covered by cracks and is rough due to the impurities and metal oxides (white impurity particles with a large variation of size 10 nm–2 μm), which became smooth and quite uniform after graphene deposition (Fig. 2f and h). The metal under the graphene coating on Mo-G and Mo/Pt-G (Fig. 2e and f, and g and h, respectively) is smooth, and the graphene coating covers the entire surface, while the Mo/Pt-G surface has better characteristics with fewer defects and irregularities. The first priority before deposition of graphene on the Mo and Mo/Pt foils is removal of the metal oxides and impurities from the surface of the bare Mo and Mo/Pt foils, since these impurities lead to two undesirable consequences during graphene growth. First, they provide nucleation sites for mono- and bi-layer graphene growth; second, they act as obstacles for growth of a continuous graphene layer [43]. The relatively smooth surface free from surface oxides of Mo foil after graphene deposition (Fig. 2e and f) can be explained by the pre-treatment process of the Mo foil before graphene deposition and to the etchant performance of H2 at a high temperature (1000 °C) in the CVD furnace prior to the growth of graphene [44]. Finally, graphene wrinkles shown in Fig. 2e originate from the different thermal expansion coefficients of Mo and graphene [45]. On the other hand, the platinum sputtered coating on Mo/Pt before immersion in 0.1 M NaCl electrolyte (Fig. 2c and d) is cracked and contains interwoven channels that can allow the NaCl corrosive electrolyte to penetrate and reach the Mo surface. The Pt thin film sputtered on Mo substrate has channeling cracks due to intrinsic stress arising during the deposition process [46], low deposition rate of the thin film [47], and biaxial in-plane tensile stresses occurring in the Pt film deposited at a high temperature and then cooled [48]. As can be seen in Fig. 2c, d channeling cracks can be observed on the Mo/Pt samples, while after 34


S. Naghdi et al.

increases again as a result of intense oxidation of Mo, followed by decreases in the barrier properties of graphene coating.

CVD of graphene (Fig. 2g, h), no cracks can be observed on the surface, suggesting that graphene coating properly covered the surface of the Mo/Pt substrate. To compare the effect of the corrosion agent on the surface morphology of the samples after prolonged exposure, Fig. 3 represents FESEM microphotographs after immersion in 0.1 M NaCl for 30 days. It is evident from Fig. 3a, that the bare Mo surface is smoothed-out due to the dissolution of the surface oxides as well as the metal itself during the exposure to 0.1 M NaCl. In the Mo/Pt, the cracks and channels are significantly wider after 30 days (Fig. 3b), with evident progression of corrosion of Mo underneath the Pt coating and destruction of the coating itself. When the Mo/Pt sample is immersed in 0.1 M NaCl, the electrolyte can enter the pre-existing cracks and channels through the Pt coating and reach the Mo substrate, where the intensive corrosion takes place. On the contrary, after 30 days of exposure to 0.1 M NaCl, the graphene-coated Mo and Mo/Pt foils (Fig. 3c and d, respectively) are still without cracks on the surface, demonstrating the ability of a graphene thin layer to protect both Mo and a Mo/Pt surface against corrosion in 0.1 M NaCl solution. However, comparing Fig. 2f and h (Mo-G and Mo/Pt-G, respectively, before exposure to 0.1 M NaCl) and Fig. 3c and d both surfaces become more granular and rough after 30 days of exposure, indicating the worsening of protective properties of graphene coating. Surface composition and elemental analysis of the bare Mo, Mo/Pt, Mo-G, and Mo/Pt-G for different times of exposure to 0.1 M NaCl were determined using EDS analysis (Table 1). All samples contain Mo and O (originating from the present oxides), along with C and Pt for coated samples. The Mo content remains relatively constant for all samples at all time periods. The bare Mo before exposure contains a large amount of oxygen, confirming that the surface of Mo is spontaneously covered with an oxide layer when exposed to ambient air. The Mo/Pt-G samples contain a slightly smaller amount of C with respect to Mo-G, in agreement with findings from Raman spectroscopy (Section 5.3), which revealed that the graphene coating on Mo/Pt-G was composed of fewer layers than that on Mo-G. The oxygen in the EDS data after graphene deposition, representing oxides formed, can be explained by oxidation of bare Mo through the defect sites of the graphene layer or graphene grain boundaries [49]. For all monitored periods, the Mo/Pt-G samples contain the smallest amount of O, indicating that the graphene coating completely covers the surface of the metal and hinders oxidation of Mo by preventing immediate contact of electrolyte with the metal. After 10 days of immersion in 0.1 M NaCl, the oxygen content decreased for all samples in comparison with 0 days, indicating further oxidation of the Mo oxide layer and formation of dissolvable species such as HMoO4− and MoO42−, in accordance with the Pourbaix diagram for Mo in neutral aqueous media [15]. After 30 days, the oxygen content

5.2. X-ray photoelectron spectroscopy To study the elemental composition of the samples, XPS was utilised (Fig. 4 and Table 2), confirming the presence of oxygen-functional groups such as C-O and COOH in the samples [50]. C1s XPS spectra of the samples were deconvoluted to show peaks assigned to different functional groups: a peak with a binding energy of 284.1–284.6 eV originates from the sp2-hybridized carbon that is typical of graphitic carbon [43,51,52], a peak at 285.9–286.2 eV originated from CeC sp3 and hydroxyl CeOH groups, and peaks at 288.3 to 288.7 eV were assigned to carbonyl C]O and carboxylic COOH groups [43,52]. Fig. 4 shows that the peak intensity, which originates from the sp2-hybridized C, is higher than the other peaks for both samples, indicating that the obtained graphene coatings have a high degree of graphitization. The presence of the oxygen-functional groups in the C1s XPS spectra of the samples might originate from contamination [52] or the adsorption of O2 after the samples were removed from the reaction chamber [44]. For Mo/Pt-G, a sharp graphitic peak sp2 (CeC) was observed at 284.4 eV, and a hydroxyl (CeOH) peak was detected at 286.7, while Mo-G showed a sharp graphitic peak sp2 at 284.2 eV, and hydroxyl (CeOH) and carbonyl C]O were observed at 285.9 and 288.3 eV, respectively [52–54]. Comparing the results of the amount of oxygen-functional groups and the full width at half maximum (FWHM) of the samples, Mo/Pt-G showed that more than 90% of the C can be ascribed to CeC sp2, while the CeC sp2 fraction is smaller than 70% for the samples of Mo-G. The Mo-G samples also had larger FWHM in comparison with Mo/Pt-G, which indicated that this sample is more oxidized and more amorphous than the Mo/Pt-G [53,54]. Furthermore, for comparison of the surface composition of the samples, Mo 3d spectra of Mo, Mo-G, and Mo/Pt-G were investigated. The peaks in the range of 224–240 eV are assigned to 3d5/2 and 3d3/2 of Mo (Fig. 5a–c). The Mo3d XPS data of Mo sample (Fig. 5a) presents two dominant peaks that are located at 232.7 and 235.8 eV, ascribed to surface-oxidized Mo foil (Mo6+) [55,56]. The Mo 3d spectrum of Mo-G sample, reveals two oxidation states of Mo2+ (228.4 eV and 231.5 eV), and Mo6+ (232.7 eV and 235.5 eV), respectively (Fig. 5b) [56,57], and for Mo/Pt-G, Mo2+ (228.4 eV and 231.5 eV), and Mo6+ (235.1 eV), respectively (Fig. 5c) [57]. As it was shown in Fig. 5b and c, the deconvoluted peaks of Mo-G and Mo/Pt-G contain molybdenum oxide peaks, suggesting that the surfaces of these samples are prone to oxidation and surface oxides are formed when they are exposed to air [58,59]. From Table 3, it can be seen that the percentage of oxidized bonds (CeOH and C]O) for Mo/Pt-G is smaller than Mo-G which is in agreement with EDS data (Table 1). Appearance of carbide peaks (Mo2+) in the XPS spectra of Mo-G and Mo/Pt-G proves that C-atoms have diffused into the metal surface. Formation of carbide phase during graphene synthesis process will lead to slowdown of the precipitation of C onto catalyst surface, acting as an obstacle for the diffusion of more C onto the surface; therefore, the deposited graphene is more uniform and thinner [32]. By comparing the Mo3d XPS data of Mo-G and Mo/Pt-G, it can be seen that the percentage of carbide peaks in Mo/Pt-G is greater than the percentage of carbide peaks in Mo-G. This is attributed to the higher solubility of C into Pt, compared to Mo [60]. Therefore, by using Pt as catalyst for graphene deposition, more carbon precipitates onto the metal surface to form carbide. Consequently, the graphene layer on this sample is thinner and has higher quality compared to Mo-G [32].

Table 1 EDS analysis for bare Mo, Mo/Pt (Pt-coated Mo foil), Mo-G (graphene-coated Mo foil) and Mo/Pt-G (graphene-coated Mo/Pt) before immersion and after 10 and 30 days of exposure to 0.1 M NaCl. Element content (at.%) bare Mo

Mo/Pt

Mo-G

Mo/Pt-G

0 days

Mo Pt C O

41.05 – – 58.95

47.86 3.74 – 48.4

52.69 – 32.52 14.79

57.2 5.99 26.1 10.71

10 days

Mo Pt C O

71.32 – – 28.68

50.2 5.3 – 44.5

48.45 – 39.02 12.53

53.32 5.08 29.79 11.81

Mo Pt C O

58.8 – – 41.2

49.81 5.68 – 44.51

50.04 – 32.15 17.81

50.46 5.73 26.95 16.86

30 days

5.3. Raman Raman spectroscopy is known as a strong tool and non-destructive method for characterization of the number of layers and quality of graphene [61]. In the Raman spectrum of graphene, the general features appear in the range of 1000–3000 cm−1. The D peak 35


S. Naghdi et al.

Fig. 4. C1s XPS spectra of (a) Mo-G (graphene-coated Mo foil), and (b) Mo/Pt-G (graphene-coated Mo/Pt).

band and G band (ID/IG) [66]. Fig. 6 shows that the D band is present in both samples, corresponding to the low crystallinity of the graphene coatings on both samples. The FWHM of the D band for Mo/Pt-G sample is greater than that for Mo-G (83.97 cm−1 and 77.54 cm−1, respectively). Furthermore, the ID/IG ratio of Mo/Pt-G is smaller than that of Mo-G, demonstrating that the graphene quality can be further improved by using Pt as catalyst for producing graphene [63]. Finally, in the Raman spectra of both Mo-G and Mo/Pt-G samples, the D + G band at about 2940 cm−1 is clearly visible. Presence of defects in the graphene layers leads to the existence of this combination scattering mode, which requires disorder for its activation [70–75]. Graphene domain size is related to the surface coverage of the graphene coating and can be calculated using the intensity ratio ID/IG and the Tuinstra-Koenig relation (Eq. (1)) [76]:

Table 2 The XPS analysis of Mo-G (graphene-coated Mo foil) and Mo/Pt-G (graphene-coated Mo/ Pt). Sample

CeC

CeOH

C]O

FWHM (eV)

Mo-G

Position Area

284.2 69.3%

285.9 23.2%

288.3 7.5%

1.75

Mo/Pt-G

Position Area

284.4 90.2%

286.7 9.8%

– –

1.64

(1350 cm−1), corresponding to the breathing modes of sp2 atoms and caused by disordered carbon atoms, is weak for highly crystalline samples [62]. The G peak (1580 cm−1) corresponds to a primary inplane vibration mode of sp2 carbon atoms, and the shape and position of this peak can be used to probe the number of graphene layers [62,63]. Moreover, the intensity of the G band increases almost linearly as the number of graphene layers increases [61,64,65]. The 2D peak (2690 cm−1), which is an overtone of the D band, always appears in the graphene Raman spectrum even when its first order peak is absent [40,62]. Shape and position of this peak are also connected to the number of graphene layers and become broader and blue shifted, respectively, when the number of graphene layers increases from single layer graphene (SLG) to multilayer graphene [61,62,66]. From the above information, it can be concluded that the ratio of peak intensities, I2D/IG, as well as the shape and position of these peaks can be used to determine the number of graphene layers. In addition, using the ratio of peak intensities ID/IG, the level of disorder and defects of graphene layers can be calculated [62]. Raman spectra for the graphene coatings (Fig. 6) were measured at different points on the samples, and the results are presented in Table 4. Table 4 shows that the 2D band of both samples exhibits a blue-shifted peak with respect to single-layer graphene (SLG at ∼2690 cm−1), while Fig. 6 illustrates that the 2D peak of Mo-G is shorter and broader than the 2D peak of Mo/Pt-G [40,58,59,61]. Therefore, it is clear that the graphene coating on both samples is not SLG. The results presented in Table 4 indicate that the graphene coating produced using Pt as catalyst consists of fewer layer graphene than graphene on Mo because the I2D/IG ratio for Mo/Pt-G (1.028) is greater than that for Mo-G (0.773). Moreover, since full width at the half maximum (FWHM) of the 2D peak increases with an increase in number of graphene layers [41], Fig. 6 shows that Mo-G has a broader 2D peak than Mo/Pt-G. In addition, it is well known that a blue shift in the 2D peak position points to an increasing number of graphene layers [41], and a greater blue shift can be observed for Mo-G in comparison to Mo/Pt-G. Finally, it is also known that carbon sheets having I2D/IG < 1 refer to few-layer graphene (FLG), while 1 < I2D/ IG < 2 refers to bi-layer graphene (BLG) [63,67–69]. Therefore, it can be concluded that Mo-G is coated by FLG, whereas Mo/Pt-G is coated by BLG [36,41,67–69]. Generally, the defects and disorders are characterized by the FWHM of the D band or the intensity ratio of the D

−1

4 ⎛ ID ⎞ Lα = 2.4 × 10−10λlaser ⎝ IG ⎠ ⎜

⎟

(1)

where Lα is the graphene domain size (in nm), and λlaser is the excitation laser wavelength (532 nm). The results in Table 4 show that graphene domain size on Mo/Pt is larger (16.8 nm) than the graphene domain size on Mo foil (11.9 nm), indicating better surface coverage of the Mo/Pt substrate. Based on all presented results, it can be concluded that the graphene coating synthesized by Pt catalyst had fewer layers, fewer defects, larger domain size, and better surface coverage compared to the graphene synthesized by Mo catalyst. 5.4. Corrosion studies 5.4.1. Open circuit potential measurements All electrochemical measurements are performed at, or starting at the open circuit potential, Eocp, of the investigated samples. Before each experiment, the samples were allowed to equilibrate, and the Eocp was measured until a stable potential was reached, with change no higher than 0.01 mV/10 s. Fig. 7 represents Eocp for bare Mo, Mo/Pt, Mo-G and Mo/Pt-G samples plotted against time of immersion in 0.1 M NaCl solution. For all the samples, the potential steadily increases with time of immersion, with steeper profile in the beginning, which slows down towards the end of the monitored time period. The bare Mo has the most negative Eocp over the entire 30 days, indicating the highest corrosion susceptibility. Both Mo-G and Mo/Pt-G exhibit more positive Eocp compared to Mo, confirming that the graphene coating makes these samples less susceptible to corrosion than the bare metal. Interestingly, Mo/Pt-G exhibits more positive Eocp than Mo-G sample in the first couple of days, but over the following 20 days the Eocp of Mo/Pt-G has more negative values. This could be due to the thinner graphene coating on the Mo/Pt-G sample, as it was found to be bi-layer from the Raman spectroscopy investigations (Section 5.3), whereas the graphene coating on Mo-G had few layers and was therefore thicker. At the end of the monitored period, the Eocp values for bare Mo, Mo-G and Mo/Pt-G 36


S. Naghdi et al.

Table 3 XPS data of Mo 3d spectrum of bare Mo foil, Mo-G (graphene-coated Mo foil), and Mo/PtG (graphene-coated Mo/Pt). Sample

Mo2+3d5/2

Mo2+3d3/2

Mo6+3d5/2

Mo6+3d3/2

Mo

Position Area

228.4 –

231.5 –

232.7 58.4%

235.8 41.6%

Mo-G

Position Area

228.4 51.4%

231.5 33.7%

232.7 12.9%

235.6 2.0%

Mo/Pt-G

Position Area

228.4 42.5%

231.5 44.0%

232.7 –

235.1 13.5%

5.4.2. Electrochemical impedance spectroscopy Impedance measurements were performed in order to investigate the corrosion performances of bare Mo, Mo/Pt, Mo-G, and Mo/Pt-G upon exposure to 0.1 M NaCl electrolyte for up to 30 days. The Nyquist plots for bare Mo, Mo/Pt, Mo-G, and Mo/Pt-G after 1 day and 14 days of exposure to 0.1 M NaCl are presented in Fig. 8a and b, respectively, while the Bode plots of the same samples for selected time periods up to 14 days and the corresponding fitting curves are presented in Fig. 8c–f. All samples exhibit the highest impedance on day 1, with decreasing trends over longer exposure times, indicating progression of corrosion. Similarly, the Bode phase angle and modulus plots all shift towards lower frequencies with increasing immersion time. The Nyquist curves (Fig. 8a and b) indicate the highest impedance values for Mo-G and the lowest for Mo/Pt and bare Mo. It is evident that Mo/Pt is much more prone to corrosion in comparison with bare Mo (Fig. 8c). This can be explained by the finding that the bare Mo surface in neutral solutions is covered by a layer of molybdenum mixed oxide composed mainly of MoO2 [8,12], which provides a slight barrier against mild corrosive environments but is susceptible to anodic dissolution. For Mo/Pt, platinum coating presumably disrupts the formation of a continuous oxide layer, and the sputter-deposited Pt film is pervaded with cracks and channels, as seen in the FE-SEM microphotograph (Fig. 2c and d). The electrolyte easily penetrates through these cracks and reaches the bare Mo surface, where galvanic corrosion freely takes place. On the other hand, Mo/Pt-G (Fig. 8f) exhibits much higher impedance and shift of Bode phase angle plot to higher frequencies in comparison with Mo/Pt, indicating significant improvement of corrosion performance. Both graphene-coated samples (Mo-G and Mo/Pt-G) exhibited improved corrosion resistance with respect to bare Mo and Mo/Pt. The barrier properties of both graphene coatings Mo-G (Fig. 8e) and Mo/Pt-G (Fig. 8f) decrease with prolonged immersion in 0.1 M NaCl, but the decrease of impedance is apparently slower in the case of Mo/Pt-G. On the other hand, the Zmod on Bode plots for Mo/Pt-G (Fig. 8f) was slightly lower than that of Mo-G (Fig. 8e), indicating that the corrosion performance of Mo/Pt-G over the first 14 days is lower than Mo-G, due to smaller thickness of bi-layer Mo/Pt-G in comparison to thicker fewlayer Mo-G, as confirmed by the Raman investigations (Section 5.3). However, using Raman spectroscopy we also found that Mo/Pt-G was generally of better quality, with larger domain size and fewer defects in respect to Mo-G. Finally, the EDS analysis (Section 5.1) confirmed the smallest degree of oxidation of Mo for the Mo/Pt-G sample, as indicated by the lowest oxygen content in this sample at all monitored time periods (Table 1). After 30 days of exposure to 0.1 M NaCl (Fig. 9), significant changes occurred on both Bode and Nyquist plots, indicating progressed corrosion of all samples. Impedance values are significantly decreased in Zmod vs. f (Fig. 9a) and in the complex plane (Fig. 9b), while the phase angle is shifted to lower frequencies (∼0.02 Hz) in comparison with the frequencies (10–100 Hz) for shorter immersion times (Fig. 8c–f). Graphene-coated Mo-G and Mo/Pt-G samples still possess higher impedance values with respect to both bare Mo and Mo/Pt. The corrosion performances of Mo-G and Mo/Pt-G are similar, however the graphene coating still remains on the Mo/Pt-G after 30 days, which can be seen

Fig. 5. Mo3d XPS spectra of (a) bare Mo foil, (b) Mo-G (graphene-coated Mo foil), and (c) Mo/Pt-G (graphene-coated Mo/Pt).

are similar, with Eocp values at 30 days of immersion in 0.1 M NaCl being −18 mV vs. SCE for both bare Mo and Mo-G, and −13 mV vs. SCE for Mo/Pt-G. The Mo/Pt sample exhibits the most positive Eocp over the entire 30 days of exposure. Platinum is the main metal in contact with electrolyte in the case of this sample; therefore the more noble values for open circuit potential are to be expected. However, in the case of Mo/Pt, Eocp cannot be taken as a sole indicator of corrosion susceptibility of the sample, as the FE-SEM images (Figs. 2c, d and 3b) showed that the surface of Pt coating is not smooth but very cracked and does not cover the entire surface of Mo. So, when the electrolyte enters these cracks, galvanic corrosion is likely to take place. 37


S. Naghdi et al.

graphene coating. This is corroborated by the significant increase in oxygen content in both Mo-G and Mo/Pt-G after 30 days, compared to 0 and 10 days, as measured by the EDS (Table 1). The obtained EIS data were fitted with equivalent electrical circuits (EEC) in order to calculate the parameters associated with the electrochemical processes on the metal/electrolyte interface. The bare Mo and Mo/Pt were modelled with a Randles-type EEC (Fig. 10a) with ohmic electrolyte resistance, Rel, coupled in series with a parallel bond of constant-phase element (CPEdl) representing double-layer capacitance and diffusion processes, and charge-transfer resistance, Rct. On the other hand, EIS spectra of Mo-G and Mo/Pt-G were modelled with EEC, as shown in Fig. 10b, where CC represents the coating capacitance of graphene layer on the Mo surface, Rp is the pore resistance of the electrolyte inside the pores of the graphene coating, and other physical quantities are as defined above. The models fitted the experimental data well, with a goodness of fit χ2 value around 10−6. Fig. 11a and b represent the time dependences of Rct and CPEdl, respectively, for all samples during the 30 days of immersion in 0.1 M NaCl. Mo/Pt-G exhibited the highest values of charge-transfer resistance (Fig. 11a) over the entire 30-day period; therefore, the graphene coating grown on Pt-coated Mo exhibits the best barrier properties. The time dependences of Rct for all investigated samples experience gradual decrease over 30 days of immersion in 0.1 M NaCl, indicating decreasing corrosion stability. However, superior resistance to corrosion is observed for Mo/Pt-G, particularly after the 16th day, when the Rct of bare Mo and Mo-G shows a sharp decline, whereas the Rct of Mo/Pt-G remains more or less constant until the end of the monitored period. In addition, the Rct of Mo/Pt is significantly lower than those of all other samples and is an entire order of magnitude lower in comparison with the Rct of Mo/Pt-G. This corroborates our assumption of the role of platinum as a catalyst for CVD synthesis of high-quality graphene coating, which provides notably better protective properties with respect to graphene coating grown directly on bare Mo surface, especially for long-term protection. The impedance of the constant phase element, ZCPE, can be represented by the following equation:

Fig. 6. Raman spectra of the graphene coatings grown on Mo (Mo-G) and on Pt-coated Mo foil (Mo/Pt-G).

Table 4 The Raman peak positions, values of ID/IG, I2D/IG and domain sizes (Lα) for Mo-G (graphene-coated Mo foil) and Mo/Pt-G (graphene-coated Mo/Pt).

Mo-G Mo/Pt-G

D (cm−1)

G (cm−1)

2D (cm−1)

ID/IG

I2D/IG

Domain size (Lα, nm)

1348.02 1350.57

1591.12 1588.43

2696.13 2692.68

1.609 1.138

0.773 1.028

11.9 16.8

ZCPE = Y 0−1 (iw )−α −1

(2) −2 α

where Y0 (Ω cm s ) is the admittance of an ideal capacitor, i2 = −1, w = 2πf is the angular frequency (rad s−1), f is the frequency in Hz, and α is an empirical constant between 0 and 1, indicating the deviance of CPE from the pure capacitor. When α is in the range between 0.8 and 1.0, ZCPE can be reasonably well approximated by the impedance of a pure capacitor, represented by Eq. (3), where C is the capacitance of an ideal capacitor. In all of our experiments, the values of α were close to or greater than 0.9; therefore, we took Y0 to represent the value of the double-layer capacitance, Cdl.

ZCPE = (iwC )−α

Fig. 7. Open circuit potential, Eocp, against time of immersion in 0.1 M NaCl for bare Mo, Mo/Pt (Pt-coated Mo foil), Mo-G (graphene-coated Mo foil) and Mo/Pt-G (graphenecoated Mo/Pt) samples.

(3)

The double-layer capacitance (Fig. 11b) expectedly increases gradually over the 30 days of exposure to 0.1 M NaCl for all samples, as electrolyte penetrates the coating and reaches the underlying metal surface. Interestingly, the Cdl of Mo/Pt-G is higher than that of Mo-G over the entire period, but this could be ascribed to the additional effect of pseudocapacitance of large-grain graphene coating and growth of a stable oxide layer on the metal surface [77,78]. The highest values of Cdl for Mo/Pt indicate the lowest corrosion stability of this sample. It should be noted that Mo-G and Mo/Pt-G impedance spectra contained only one capacitive contribution after 20 days (fitted by EEC in Fig. 10a), corresponding to the pseudocapacitance of the double layer on the Mo surface. This attests that both graphene coatings experienced decreasing barrier properties, allowing for penetration of the electrolyte and the onset of unobstructed corrosion of the underlying metal. However, the Rct values remain highest for Mo/Pt-G samples, indicating that the graphene coating on Mo/Pt-G retains the most protective properties and provides the best barrier to Mo corrosion during

from shoulder in Bode phase angle plot between 100 and 1000 Hz (Fig. 9a), as well as from small high frequency semicircle in the Nyquist plot (Fig. 9b). On the contrary, the graphene coating cannot be observed at Mo-G (absence of small high frequency semicircle in the Nyquist plot). Mo/Pt still exhibits the lowest impedance. We assume that, during the CVD of graphene onto the Mo/Pt surface, some dewetting of the Pt film occurs, during which the Pt coating shrinks and recedes, increasing the surface area covered with cracks and channels. This leaves the oxide-free molybdenum surface inducing better adhesion of graphene coating, as indicated by the lower oxygen content in Mo/Pt-G samples, obtained from EDS analysis (Section 5.1, Table 1). Upon prolonged exposure to 0.1 M NaCl, a fresh layer of Mo oxide is allowed to grow, providing a synergistic effect of Mo oxide and

38


S. Naghdi et al.

Fig. 8. Nyquist plots of bare Mo, Mo/Pt (Pt-coated Mo foil), Mo-G (graphene-coated Mo foil) and Mo/Pt-G (graphene-coated Mo/Pt) after (a) 1 and (b) 14 days of immersion in 0.1 M NaCl; Bode plots of (c) bare Mo, (d) Mo/Pt, (e) Mo-G and (f) Mo/Pt-G after different times of immersion in 0.1 M NaCl; 10 mV amplitude around Eocp, frequency range of 100 kHz–10 mHz.

underneath. This was also indicated by the Eocp measurements (Section 5.4.1), where the Eocp of Mo/Pt-G exhibited more negative values than that of Mo-G between 5 and 25 days of exposure, presumably because of the smaller thickness of Mo/Pt-G coating. The better quality coating should also provide better protection, but smaller thickness of the graphene coating affects negatively its protective properties. Therefore, subject of future work will be focused on attempting to produce highquality graphene coatings using Pt as a catalyst for the CVD, but consisting of few graphene layers, in order to improve their protective properties.

prolonged exposure to 0.1 M NaCl solution. In conclusion, the results of the EIS investigations indicate that the graphene coatings provided corrosion protection for both Mo-G and Mo/Pt-G, in comparison with non-coated samples (bare Mo and Mo/Pt) in 0.1 M NaCl. Although the EIS results did not indicate improved corrosion performances of Mo/Pt-G over the Mo-G during initial time of exposure, after long time Mo/Pt-G still retained its protective properties while Mo-G exhibited coating loss. The Raman spectroscopy results have shown that, even though the graphene coating Mo/Pt-G is of higher quality, with larger grain size and with fewer defects than Mo-G, it is also of lower thickness (bi-layer), compared to few-layers Mo-G. Since the barrier properties of graphene coating depend not only on the quality of the coating, but also on its thickness, the graphene bi-layer Mo/Pt-G might not provide sufficient barrier to electrolyte for the metal

5.4.3. Potentiodynamic sweep (PDS) and linear polarization (LPR) In addition to EIS measurements, the polarization curves were recorded at designated periods of immersion in 0.1 M NaCl in order to 39


S. Naghdi et al.

Fig. 9. Impedance spectra of bare Mo, Mo/Pt (Pt-coated Mo foil), Mo-G (graphene-coated Mo foil), and Mo/Pt-G (graphene-coated Mo/Pt) after 30 days of immersion in 0.1 M NaCl; 10 mV amplitude around Eocp, frequency range of 100 kHz–10 mHz. Fig. 10. Equivalent electrical circuits used to model the impedance spectra of (a) bare Mo and Mo/Pt (Ptcoated Mo foil), (b) Mo-G (graphene-coated Mo foil) and Mo/Pt/G (graphene-coated Mo-Pt).

throughout the entire monitored period, with observed trends of decrease in Rp and increases in jcorr and vcorr. These results are in good agreement with EIS findings and confirm a significant decrease in corrosion stability of Mo upon sputter coating with Pt. Likely, this is due to rupture of natural oxide film and galvanic corrosion in the cracks and channels of Pt coating where bare Mo is exposed and tends to corrode intensively in the presence of electrolyte, being the less noble metal in contact. The Ecorr for Mo/Pt is shifted towards more positive values in comparison with the bare Mo (Fig. 12a), due to the Pt coating. Similarly, the Ecorr for both Mo-G and Mo/Pt-G samples (Fig. 12a) are shifted towards more noble values than the bare Mo, indicating the decrease in anodic dissolution and increase in corrosion resistance due to the presence of graphene coating. When Mo/Pt is coated with CVDgrown graphene, the Mo/Pt-G exhibits the lowest corrosion rates and highest polarization resistance. This is obvious from the data listed in Table 5, where, upon initial exposure to 0.1 M NaCl, the value of Rp for Mo/Pt-G (53.5 Ω cm2) is nearly two times higher than those for bare Mo and Mo-G (19.5 Ω cm2 and 29.7 Ω cm2, respectively). After 20 and 30 days, the corrosion current densities for bare Mo and Mo-G are comparable, whereas jcorr values for Mo/Pt-G remain two times lower. The calculated values for jcorr, vcorr, and Ecorr of bare Mo upon initial immersion in 0.1 M NaCl (0.615 μA cm−2, 0.10 pm s−1, and −224 mV vs. SCE, respectively, Table 5) are in good agreement with previously reported data [24,81]. The corrosion rate of bare Mo exhibits a significant increase upon 10 days of exposure (6.52 μA cm−2); after 20 days, it decreases to 2.22 μA cm−1, and then starts to slowly increase until the end of monitored period. This could be related to the formation of a mixed oxide layer, which provides a slight short-term protection to the metal underneath but dissolves upon prolonged exposure due to insufficient thermodynamic stability in neutral solutions [82]. A similar trend is observed for the corrosion rates of Mo-G and Mo/Pt-G over this period. The corrosion rate of Mo-G is lower, but always close to that of bare Mo; however, Mo/Pt-G exhibits the lowest jcorr and vcorr, confirming the role of Pt coating as a catalyst for CVD growth of large-

quantitatively evaluate the kinetic parameters of corrosion. The polarization curves for bare Mo, Mo/Pt, Mo-G, and Mo/Pt-G, obtained in the range of ± 150 mV with respect to EOCP, after 30 min and 10, 20, and 30 days of exposure to 0.1 M NaCl are presented in Fig. 12. The corrosion potential, Ecorr, and corrosion current density, jcorr, can be evaluated using Tafel extrapolation for the linear region of anodic or cathodic polarization curves [79], at potentials far from Ecorr. The anodic components of all polarization curves in Fig. 12 were satisfactorily linear over at least a decade of current, so we used the anodic curves for evaluation of Ecorr and jcorr. Linear regions of anodic and cathodic curves were used to evaluate anodic and cathodic Tafel slopes, ba and bc, respectively. Polarization resistance, Rp, a measure of corrosion stability, was calculated from the slope of linear polarization curves recorded in the potential range of ± 5 mV vs. Eocp. Finally, we calculated the corrosion rates in linear units, vcorr, from the obtained corrosion current density values, using the following equation:

vcorr =

M j nFρ corr

(4) −1

where M is the molar mass of the metal (95.96 g mol for Mo), n is the number of electrons exchanged in the corrosion reaction, F is the Faraday constant, and ρ is the density of Mo (10.28 g cm−3). The values of all calculated parameters (polarization resistance, corrosion potential, corrosion current density, Tafel slopes and corrosion rate) are listed in Table 5 for bare Mo, Mo/Pt, Mo-G, and Mo/Pt-G after 30 min, 10, 20, and 30 days of exposure to 0.1 M NaCl electrolyte. No passive or transpassive regions were observed for any of the anodic polarization curves in Fig. 12. This is in agreement with expected behaviour in neutral and basic solutions, which are generally more aggressive towards Mo than acidic environments [7,80]; therefore, no passivation layer is formed on the surface, and only active dissolution of the metal is observed in the applied potential range. Upon initial immersion (30 min, Table 5), Mo/Pt exhibits the highest corrosion rates and the lowest polarization resistance, and this is consistent 40


S. Naghdi et al.

investigate the oxidation and reduction processes on the Mo surface and the influence of different coatings. The Eocp was allowed to reach the equilibrium value, and CVs were recorded after around 30 min. Fig. 13 represents stationary CVs (the cycling was continued until steady state was reached) for all samples. The cyclic voltammogram of bare Mo (Fig. 13a) contains usual features, described in our previous work [24]. In brief, the anodic sweep contains a wide oxidation wave around −300 mV vs. SCE, associated with the oxidation of Mo to MoO2 and formation of an oxide layer on the metal surface [12], and the corresponding cathodic peak of oxide reduction is located around −500 mV vs. SCE. Another intense anodic wave is observed close to 0.2 V vs. SCE and ascribed to the oxidation of MoO2 to MoO42− and HMoO4− [24,82]. In the Mo-G and Mo/Pt-G samples (Fig. 13a), the graphene coatings seem to hinder the oxidation of Mo surface and formation of oxide layer, as seen by the apparent decrease of anodic current densities of associated peaks, as well as by the shift of the peak potentials to more positive values (−140 mV vs. SCE for Mo-G and −120 mV vs. SCE for Mo/Pt-G). This confirms that the graphene coating acts as a barrier to electrolyte and provides protection against oxidation of the metal underneath. Moreover, the general appearance of the CVs of coated samples did not change with respect to bare Mo, and no additional processes were observed, indicating that the graphene layer is stable and remains unaffected in the scanned potential range. The anodic and cathodic peaks are slightly more pronounced in Mo/Pt-G, probably due to the presence of platinum, which increases the electroactivity of the metal surface. Additionally, it is worth noting that the current density values for the first anodic peak for Mo-G and Mo/Pt-G samples only slightly decreased after the first cycle (Fig. 13b); during the subsequent cycles, the steady state is quickly reached, indicating that some change of the surface properties occurs upon initial cycling. The change in anodic current density between the first and second cycles for bare Mo is not as evident as that observed in acid-buffered NaCl solution [7]. Therefore, in the neutral NaCl solution utilized in our work, there is likely no significant formation of a passive layer on the Mo surface, and deposited MoO2 film is readily further oxidized upon increasing the cycling potential to 0.2 V vs. SCE. The CV of Mo/Pt (Fig. 13c) exhibits significant differences in comparison with all other samples. The current densities in both anodic and cathodic sweeps are higher due to the presence of a Pt layer. The CV is similar to that of a bulk Pt electrode. In the cathodic region, the most prominent feature is a peak at −1.0 V vs. SCE, corresponding to hydrogen adsorption on the Pt electrode. In the anodic sweep, the first wave around −0.9 V vs. SCE involves desorption of hydrogen and H2 evolution. This peak is hindered compared to the CV of a Pt electrode [83] and does not contain usual characteristics with two or three peaks as a result of a significantly smaller active surface area of sputter-deposited Pt film. A weak oxidation peak is still visible around −0.2 V vs. SCE, previously ascribed to molybdenum oxidation. The sharp oxidation peak at 0.2 V vs. SCE probably involves only further oxidation of MoO2 and not formation of Pt oxides, as there was no observed reduction peak in the cathodic sweep that could be ascribed to their reduction. Finally, comparing the CV of Mo/Pt-G (Fig. 13a) to that of Mo/ Pt (Fig. 13c), there is an evident decrease in current density as well as disappearance of features connected to Pt, which indicates that the Pt film was dewetted to some extent during the CVD, and that the graphene coating provides full coverage of the surface and acts as a barrier to oxidation of underlying metal.

Fig. 11. Time dependence of (a) charge-transfer resistance, Rct, and (b) double-layer capacitance, Cdl, for bare Mo, Mo/Pt (Pt-coated Mo foil), Mo-G (graphene-coated Mo foil) and Mo/Pt-G (graphene-coated Mo/Pt), calculated from the EIS spectra obtained for different times of immersion in 0.1 M NaCl (up to 30 days); 10 mV amplitude around Eocp, frequency range of 100 kHz–10 mHz.

grain graphene coating with better barrier properties compared to graphene grown directly on bare Mo. The Pt as a catalyst enables the synthesis of large-grain, good quality bi-layer graphene coating on Mo, and therefore thinner than few-layer graphene coating on Mo without Pt, as confirmed by the Raman spectroscopy (Section 5.3). Table 5 shows that the anodic Tafel slopes, ba, for Pt-coated samples (Mo/Pt and Mo/Pt-G) are somewhat lower (around 120 mV dec−1) in comparison with bare Mo and Mo-G (150–160 mV dec−1), indicating that the presence of Pt influences the mechanism of the anodic reaction. The ba values do not change significantly over time, suggesting that the anodic dissolution of Mo progresses according to the same mechanism during the observed period. On the other hand, the values of cathodic Tafel slopes, bc, exhibit notable decreases with time, which could be the consequence of an oxygen reduction reaction mechanism change on the surface of Mo due to adsorption of different species and alteration of the surface state.

6. Conclusions In this work, graphene was deposited on Mo and Pt-coated Mo foils via a chemical vapour deposition technique at atmospheric pressure (APCVD). The results of EDS analysis showed that the graphene-coated Mo/Pt (Mo/Pt-G) contained the smallest amounts of oxygen, indicating that the surface is completely covered by the graphene coating. This was corroborated by the findings from XPS that the surface of Mo/Pt-G

5.4.4. Cyclic voltammetry We recorded cyclic voltammograms (CV) for bare Mo, Mo/Pt, Mo-G, and Mo/Pt-G upon initial immersion in 0.1 M NaCl in order to 41


S. Naghdi et al.

Fig. 12. Polarization curves of bare Mo, Mo/Pt (Pt-coated Mo foil), Mo-G (graphene-coated Mo foil) and Mo/Pt-G (graphene-coated Mo/Pt) after (a) 30 min, (b) 10 days, (c) 20 days and (d) 30 days of immersion in 0.1 M NaCl; potential range ± 150 mV vs. SCE with respect to Eocp.

Mo-G, but after long time of exposure graphene coating Mo/Pt-G still remained on the surface, whereas graphene coating Mo-G completely disappeared. In addition, the charge-transfer resistance values for Mo-G and Mo/Pt-G were similar for the initial period of exposure to corrosive electrolyte, while during prolonged exposure the Rct values of Mo/Pt-G were greater than of Mo/Pt-G. The results of PDS measurements showed that the graphene coating on Pt-coated Mo exhibited the highest values of polarization resistance and the lowest values of corrosion current density and corrosion rate, indicating lower

is less amorphous and less oxidized than that of Mo-G. Mo3d peaks revealed greater percentage of carbide peaks in Mo/Pt-G, pointing that the graphene coating on Mo/Pt-G is thinner and of better quality compared to Mo-G. Furthermore, formation of a thin bi-layer graphene coating with fewer defects and larger domain size on Pt-coated Mo was confirmed by Raman results, while the graphene coating synthesized on Mo (Mo-G) was few-layer with more defects and smaller domain size. EIS measurements for short-time period of immersion in 0.1 M NaCl proved slightly lower impedance of graphene coating Mo/Pt-G than

Table 5 Calculated corrosion parameters for bare Mo, Mo/Pt, Mo-G and Mo/Pt-G after 30 min, 10, 20 and 30 days of immersion in 0.1 M NaCl, obtained by Tafel analysis in the potential range of ± 150 mV/SCE from the Eocp. immersion time

sample

Rp (kΩ cm2)

Ecorr (mV vs. SCE)

jcorr (μA cm−2)

ba (V dec−1)

−bc (V dec−1)

vcorr (pm s−1)

30 min

bare Mo Mo/Pt Mo-G Mo/Pt-G

19.5 1.92 29.7 53.5

−224 −41 −100 −57.7

0.615 6.91 0.568 0.433

0.164 0.118 0.151 0.117

0.164 0.104 0.104 0.187

0.10 1.11 0.09 0.07

10 days


2.44 2.05 3.47 3.90

−68 −92 −62 −74

6.52 5.20 4.25 3.99

0.129 0.133 0.128 0.124

0.146 0.080 0.118 0.140

1.05 0.84 0.69 0.64

20 days


7.78 1.66 8.23 9.28

−99.3 −66 −77.9 −124

2.22 4.79 2.18 1.11

0.118 0.113 0.116 0.123

0.162 0.066 0.145 0.074

0.36 0.77 0.35 0.18

30 days


2.82 1.19 3.96 4.35

−110 −115 −110 −114

3.23 6.32 3.20 1.98

0.135 – 0.142 0.147

0.048 0.014 0.021 0.015

0.52 1.02 0.52 0.20

42


S. Naghdi et al.

Fig. 13. Cyclic voltammograms of (a) bare Mo, Mo-G (graphene-coated Mo foil) and Mo/Pt-G (graphene-coated Mo/Pt); (b) first cycle of CVs for bare Mo, Mo-G and Mo/Pt-G; and (c) CV of Mo/Pt (Pt-coated Mo foil) after 30 min of immersion in 0.1 M NaCl; potential range −1.0 V to 0.2 V vs. SCE, starting from Eocp, scan rate 20 mV/s.

susceptibility to corrosion of Mo/Pt-G coating. The results of this study confirmed the role of Pt thin film as a catalyst for CVD synthesis of highquality protective graphene coatings.

[9] [10]

Acknowledgements

[11]

This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (project number: 2016R1A2B4016034).

[12] [13] [14]

References

[15]

[1] F. Xie, W.C.H. Choy, C. Wang, X. Li, S. Zhang, J. Hou, Low-temperature solutionprocessed hydrogen molybdenum and vanadium bronzes for an efficient holetransport layer in organic electronics, Adv. Mater. 25 (2013) 2051. [2] J. Griffin, A.J. Pearson, N.W. Scarratt, T. Wang, D.G. Lidzey, A.R. Buckley, Organic photovoltaic devices incorporating a molybdenum oxide hole-extraction layer deposited by spray-coating from an ammonium molybdate tetrahydrate precursor, Org. Electron. 15 (2014) 692. [3] D. An, Y. Shen, J. Wen, Z. Zheng, J. Chen, J. She, H. Chen, S. Deng, N. Xu, Molybdenum nanoscrews a novel non-coinage-metal substrate for surface-enhanced raman scattering, Nano-Micro Lett. 9 (2017) 1. [4] N. Maheswari, G. Muralidharan, Controlled synthesis of nanostructured molybdenum oxide electrodes for high performance supercapacitor devices, Appl. Surf. Sci. 416 (2017) 461. [5] D. Sun, S. Xu, L. Zhang, C. Wei, Y. Li, Y. Chi, Y. Zhao, X. Zhang, Improve the conversion efficiency of Cu2 ZnSnSe4 solar cells using a novel molybdenum back contact structure, Appl. Surf. Sci. 413 (2017) 408. [6] M. Gaillard, M. Virginie, A.Y. Khodakov, New molybdenum-based catalysts for dry reforming of methane in presence of sulfur: a promising way for biogas valorization, Catal. Today 289 (2016) 143. [7] M.N. Hull, On the anodic dissolution of molybdenum in acidic and alkaline electrolytes, J. Electroanal. Chem. 38 (1972) 143. [8] M. Petrova, M. Bojinov, S. Zanna, P. Marcus, Mechanism of anodic oxidation of

[16]

[17]

[18]

[19]

[20] [21] [22]

43

molybdenum in nearly-neutral electrolytes studied by electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy, Electrochim. Acta 56 (2011) 7899. M. Bojinov, I. Betova, R. Raicheff, Transpassivity of molybdenum in H2SO4 solution, J. Electroanal. Chem. 381 (1995) 123. W.A. Badawy, F.M. Al-Kharafi, Corrosion and passivation behaviors of molybdenum in aqueous solutions of different pH, Electrochim. Acta 44 (1998) 693. C.R. Tomachuk, L. De Rosa, J. Springer, D.B. Mitton, S. Saiello, F. Bellucci, The wet corrosion of molybdenum thin film part ll: behavior at 85C, Mater. Corros 55 (2004) 665. K. Wang, Y.-S. Li, P. He, In situ identification of surface species on molybdenum in different media, Electrochim. Acta 43 (1998) 2459. M. Ürgen, U. Stolz, R. Kirchheim, ESCA measurements of films on molybdenum formed in the passive and transpassive region, Corros. Sci. 30 (1990) 377. I.A. Okonkwo, J. Doff, A. Baron-Wiecheć, G. Jones, E.V. Koroleva, P. Skeldon, G.E. Thompson, Oxidation states of molybdenum in oxide films formed in sulphuric acid and sodium hydroxide, Thin Solid Films 520 (2012) 6318. N. Takeno, Atlas of Eh-pH diagrams Intercomparison of thermodynamic databases. Geological Survey of Japan Open File Report No. 419, National Institute of Advanced Industrial Science and Technology Research Center for Deep Geological Environments, Tokyo, Japan, 2005, p. 149. R.D. Armstrong, M.F. Bell, A.A. Metcalfe, The anodic dissolution of molybdenum in alkaline solutions—X-ray photoelectron spectroscopic measurements, J. Electroanal. Chem. 84 (1977) 73. R.D. Armstrong, M.F. Bell, A.A. Metcalfe, The anodic dissolution of molybdenum in alkaline solutions—electrochemical measurements, J. Electroanal. Chem. 84 (1977) 61. S. Naghdi, B. Jaleh, A. Ehsani, Electrophoretic deposition of graphene oxide on aluminum: characterization, low thermal annealing, surface and anticorrosive properties, Bull. Chem. Soc. Jpn. 88 (2015) 722. S.C. Sahu, A.K. Samantara, M. Seth, S. Parwaiz, B.P. Singh, P.C. Rath, B.K. Jena, A facile electrochemical approach for development of highly corrosion protective coatings using graphene nanosheets, Electrochem. Commun. 32 (2013) 22. X.-W. Liu, J.-J. Mao, P.-D. Liu, X.-W. Wei, Fabrication of metal-graphene hybrid materials by electroless deposition, Carbon 49 (2011) 477. S. Qin, X. Guo, Y. Cao, Z. Ni, Q. Xu, Strong ferromagnetism of reduced graphene oxide, Carbon 78 (2014) 559. Z.-Y. Juang, C.-Y. Wu, C.-W. Lo, W.-Y. Chen, C.-F. Huang, J.-C. Hwang, F.-R. Chen, K.-C. Leou, C.-H. Tsai, Synthesis of graphene on silicon carbide substrates at low


S. Naghdi et al.

[54] D. Yang, A. Velamakanni, G. Bozoklu, S. Park, M. Stoller, R.D. Piner, S. Stankovich, I. Jung, D.A. Field, C.A. Ventrice, Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and Micro-Raman spectroscopy, Carbon 47 (2009) 145. [55] M. Chen, J. Liu, W. Zhou, J. Lin, Z. Shen, Nitrogen-doped graphene-supported transition-metals carbide electrocatalysts for oxygen reduction reaction, Sci. Rep.—UK 5 (2015) 10389. [56] C. Lu, D. Tranca, J. Zhang, F.R. Hernández, Y. Su, X. Zhuang, F. Zhang, G. Seifert, X. Feng, Molybdenum carbide-embedded nitrogen-doped porous carbon nanosheets as electrocatalysts for water splitting in alkaline media, ACS Nano 11 (2017) 3933. [57] X. Yang, X. Feng, H. Tan, H. Zang, X. Wang, Y. Wang, E. Wang, Y. Li, N-Doped graphene-coated molybdenum carbide nanoparticles as highly efficient electrocatalysts for the hydrogen evolution reaction, J. Mater. Chem. A 4 (2016) 3947. [58] C. He, J. Tao, Exploration of the electrochemical mechanism of ultrasmall multiple phases molybdenum carbides nanocrystals for hydrogen evolution reaction, RSC Adv. 6 (2016) 9240. [59] J. Halim, S. Kota, M.R. Lukatskaya, M. Naguib, M.-Q. Zhao, E.J. Moon, J. Pitock, J. Nanda, S.J. May, Y. Gogotsi, M.W. Barsoum, Synthesis and characterization of 2D molybdenum carbide (MXene), Adv. Funct. Mater. 26 (2016) 3118. [60] S.M. Irving, P.L. Walker, Interaction of evaporated carbon with heated metal substrates, Carbon 5 (1967) 399. [61] Z. Ni, Y. Wang, T. Yu, Z. Shen, Raman spectroscopy and imaging of graphene, Nano Res. 1 (2008) 273. [62] I. Childres, L.A. Jauregui, W. Park, H. Cao, Y.P. Chen, Raman spectroscopy of graphene and related materials, in: J.I. Jang (Ed.), New Developments in Photon and Materials Research, Nova Science Publishers, Hauppauge, NY, 2013. [63] Z. Wang, R. Zhang, Z. Zhang, Z. Huang, C. Liu, D. Fu, J. Liu, Raman spectroscopy of few-layer graphene prepared by C2-C6 cluster ion implantation, Nucl. Instrum. Meth. B 307 (2013) 40. [64] Y.Y. Wang, Z.H. Ni, T. Yu, Z.X. Shen, H.M. Wang, Y.H. Wu, W. Chen, A.T. Shen Wee, Raman studies of monolayer graphene: the substrate effect, J. Phys. Chem. C 112 (2008) 10637. [65] A. Ferrari, J. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. Novoselov, S. Roth, Raman spectrum of graphene and graphene layers, Phys. Rev. Lett. 97 (2006) 187401. [66] D.S. Lee, C. Riedl, B. Krauss, K. von Klitzing, U. Starke, J.H. Smet, Raman spectra of epitaxial graphene on SiC and of epitaxial graphene transferred to SiO2, Nano Lett. 8 (2008) 4320. [67] Z. Tu, Z. Liu, Y. Li, F. Yang, L. Zhang, Z. Zhao, C. Xu, S. Wu, H. Liu, H. Yang, Controllable growth of 1-7 layers of graphene by chemical vapour deposition, Carbon 73 (2014) 252. [68] Z. Lin, X. Ye, J. Han, Q. Chen, P. Fan, H. Zhang, D. Xie, H. Zhu, M. Zhong, Precise control of the number of layers of graphene by picosecond laser thinning, Sci. Rep.—UK 5 (2015) 11662. [69] W. Wu, Z. Liu, L.A. Jauregui, Q. Yu, R. Pillai, H. Cao, J. Bao, Y.P. Chen, S.-S. Pei, Wafer-scale synthesis of graphene by chemical vapor deposition and its application in hydrogen sensing, Sens. Actuators B—Chem. 150 (2010) 296. [70] D. Ye, S.-Q. Wu, Y. Yu, L. Liu, X.-P. Lu, Y. Wu, Patterned graphene functionalization via mask-free scanning of micro-plasma jet under ambient condition, Appl. Phys. Lett. 104 (2014) 103105. [71] G. Khurana, N. Kumar, R. Kotnala, T. Nautiyal, R. Katiyar, Temperature tuned defect induced magnetism in reduced graphene oxide, Nanoscale 5 (2013) 3346. [72] Y. Dong, Z. Zhang, Y. Xia, Y.-S. Chui, J.-M. Lee, J.A. Zapien, Green and facile synthesis of Fe 3 O 4 and graphene nanocomposites with enhanced rate capability and cycling stability for lithium ion batteries, J. Mater. Chem. A 3 (2015) 16206. [73] M. Iqbal, A.K. Singh, M. Iqbal, J. Eom, Raman fingerprint of doping due to metal adsorbates on graphene, J. Phys.—Condens. Matter 24 (2012) 335301. [74] N. Gogneau, A.B.G. Trabelsi, M.G. Silly, M. Ridene, M. Portail, A. Michon, M. Oueslati, R. Belkhou, F. Sirotti, A. Ouerghi, Investigation of structural and electronic properties of epitaxial graphene on 3C–SiC (100)/Si (100) substrates, Nanotechnol. Sci. Appl. 7 (2014) 85. [75] D.C. Elias, R.R. Nair, T. Mohiuddin, S. Morozov, P. Blake, M. Halsall, A. Ferrari, D. Boukhvalov, M. Katsnelson, A. Geim, Control of graphene's properties by reversible hydrogenation: evidence for graphane, Science 323 (2009) 610. [76] X. Li, C.W. Magnuson, A. Venugopal, J. An, J.W. Suk, B. Han, M. Borysiak, W. Cai, A. Velamakanni, Y. Zhu, Graphene films with large domain size by a two-step chemical vapor deposition process, Nano Lett. 10 (2010) 4328. [77] V.S. Saji, C.W. Lee, Potential and pH dependent pseudocapacitance of Mo/Mo oxides—an impedance study, Electrochim. Acta 137 (2014) 647. [78] M.D. Stoller, C.W. Magnuson, Y. Zhu, S. Murali, J.W. Suk, R. Piner, R.S. Ruoff, Interfacial capacitance of single layer graphene, Energ. Environ. Sci. 4 (2011) 4685. [79] E. McCafferty, Validation of corrosion rates measured by the Tafel extrapolation method, Corros. Sci. 47 (2005) 3202. [80] L.D. Rosa, C.R. Tomachuk, J. Springer, D.B. Mitton, S. Saiello, F. Bellucci, The wet corrosion of molybdenum thin film—part I: behavior at 25C, Mater. Corros. 55 (2004) 602. [81] V.S. Shaldaev, A.N. Malofeeva, A.D. Davydov, Determination of corrosion rate of molybdenum, rhenium and their alloys in sodium chloride solution by the method of Tafel extrapolation, Russ. J. Electrochem. 50 (2014) 994. [82] F.J. Presuel-Moreno, M.A. Jakab, J.R. Scully, Inhibition of the oxygen reduction reaction on copper with cobalt, cerium, and molybdate ions, J. Electrochem. Soc. 152 (2005) B376. [83] D.A.J. Rand, R. Woods, A study of the dissolution of platinum, palladium, rhodium and gold electrodes in 1 m sulphuric acid by cyclic voltammetry, J. Electroanal. Chem. 35 (1972) 209.

temperature, Carbon 47 (2009) 2026. [23] Y. Zhu, S. Murali, W. Cai, X. Li, J.W. Suk, J.R. Potts, R.S. Ruoff, Graphene and graphene oxide: synthesis, properties, and applications, Adv. Mater. 22 (2010) 3906. [24] S. Naghdi, I. Jevremović, V. Mišković-Stanković, K.Y. Rhee, Chemical vapour deposition at atmospheric pressure of graphene on molybdenum foil: effect of annealing time on characteristics and corrosion stability of graphene coatings, Corros. Sci. 113 (2016) 116. [25] S. Naghdi, K.Y. Rhee, S.J. Park, Oxidation resistance of graphene-coated molybdenum: effects of pre-washing and hydrogen flow rate, Int. J. Refract. Met. Hard Mater. 65 (2016) 29. [26] N. Kirkland, T. Schiller, N. Medhekar, N. Birbilis, Exploring graphene as a corrosion protection barrier, Corros. Sci. 56 (2012) 1. [27] A.S. Kousalya, A. Kumar, R. Paul, D. Zemlyanov, T.S. Fisher, Graphene: an effective oxidation barrier coating for liquid and two-phase cooling systems, Corros. Sci. 69 (2013) 5. [28] V. Mišković-Stanković, I. Jevremović, I. Jung, K. Rhee, Electrochemical study of corrosion behavior of graphene coatings on copper and aluminum in a chloride solution, Carbon 75 (2014) 335. [29] Z. Yang, W. Sun, L. Wang, S. Li, T. Zhu, G. Liu, Liquid-phase exfoliated fluorographene as a two dimensional coating filler for enhanced corrosion protection performance, Corros. Sci. 103 (2016) 312. [30] C.F. Glover, C. Richards, J. Baker, G. Williams, H.N. McMurray, In-coating graphene nano-platelets for environmentally-friendly corrosion protection of iron, Corros. Sci. 114 (2017) 169. [31] S. Liu, L. Gu, H. Zhao, J. Chen, H. Yu, Corrosion resistance of graphene-reinforced waterborne epoxy coatings, J. Mater. Sci. Technol. 32 (2016) 425. [32] S. Naghdi, K.Y. Rhee, M.T. Kim, B. Jaleh, S.J. Park, Atmospheric chemical vapor deposition of graphene on molybdenum foil at different growth temperatures, Carbon Lett. 18 (2016) 37. [33] Y. Dong, Q. Liu, Q. Zhou, Time-dependent protection of ground and polished Cu using graphene film, Corros. Sci. 90 (2015) 69. [34] I. Wlasny, P. Dabrowski, M. Rogala, P.J. Kowalczyk, I. Pasternak, W. Strupinski, J.M. Baranowski, Z. Klusek, Role of graphene defects in corrosion of graphenecoated Cu(111) surface, Appl. Phys. Lett. 102 (2013) 111601. [35] I. Wlasny, P. Dabrowski, M. Rogala, I. Pasternak, W. Strupinski, J.M. Baranowski, Z. Klusek, Impact of electrolyte intercalation on the corrosion of graphene-coated copper, Corros. Sci. 92 (2015) 69. [36] H.U. Rashid, K. Yu, M.N. Umar, M.N. Anjum, K. Khan, N. Ahmad, M.T. Jan, Catalyst role in chemical vapor deposition (CVD) process: a review, Rev. Adv. Mater. Sci. 40 (2015) 235. [37] C.-D. Liao, Y.-Y. Lu, S.R. Tamalampudi, H.-C. Cheng, Y.-T. Chen, Chemical vapor deposition synthesis and Raman spectroscopic characterization of large-area graphene sheets, J. Phys. Chem. A 117 (2013) 9454. [38] V. Yong, H.T. Hahn, Graphene growth with giant domains using chemical vapor deposition, CrystEngComm 13 (2011) 6933. [39] J. Sun, Y. Nam, N. Lindvall, M.T. Cole, K.B. Teo, Y.W. Park, A. Yurgens, Growth mechanism of graphene on platinum: surface catalysis and carbon segregation, Appl. Phys. Lett. 104 (2014) 152107. [40] J. Ping, M.S. Fuhrer, Carbon impurities on graphene synthesized by chemical vapor deposition on platinum, J. Appl. Phys. 116 (2014) 044303. [41] S. Karamat, S. Sonuşen, Ü. Çelik, Y. Uysallı, E. Özgönül, A. Oral, Synthesis of few layer single crystal graphene grains on platinum by chemical vapour deposition, Prog. Nat. Sci. 25 (2015) 291. [42] G.W. Cushing, V. Johánek, J.K. Navin, I. Harrison, Graphene growth on Pt (111) by ethylene chemical vapor deposition at surface temperatures near 1000 K, J. Phys. Chem. C 119 (2015) 4759. [43] S.M. Kim, A. Hsu, Y.-H. Lee, M. Dresselhaus, T. Palacios, K.K. Kim, J. Kong, The effect of copper pre-cleaning on graphene synthesis, Nanotechnology 24 (2013) 365602. [44] S. Wang, Y. Pei, X. Wang, H. Wang, Q. Meng, H. Tian, X. Zheng, W. Zheng, Y. Liu, Synthesis of graphene on a polycrystalline Co film by radio-frequency plasma-enhanced chemical vapour deposition, J. Phys. D Appl. Phys. 43 (2010) 455402. [45] Y. Zhang, L. Zhang, C. Zhou, Review of chemical vapor deposition of graphene and related applications, Acc. Chem. Res. 46 (2013) 2329. [46] H. Windischmann, Intrinsic stress in sputter-deposited thin films, Crit. Rev. Solid State 17 (1992) 547. [47] B. Kemmenoe, G. Bullock, Structure analysis of sputter-coated and ion-beam sputter-coated films: a comparative study, J. Microsc. 132 (1983) 153. [48] Z.C. Xia, J.W. Hutchinson, Crack patterns in thin films, J. Mech. Phys. Solids 48 (2000) 1107. [49] S. Chen, L. Brown, M. Levendorf, W. Cai, S.-Y. Ju, J. Edgeworth, X. Li, C.W. Magnuson, A. Velamakanni, R.D. Piner, Oxidation resistance of graphenecoated Cu and Cu/Ni alloy, ACS Nano 5 (2011) 1321. [50] W. Yang, C. He, L. Zhang, Y. Wang, Z. Shi, M. Cheng, G. Xie, D. Wang, R. Yang, D. Shi, Growth, characterization, and properties of nanographene, Small 8 (2012) 1429. [51] G. Kalita, M.E. Ayhan, S. Sharma, S.M. Shinde, D. Ghimire, K. Wakita, M. Umeno, M. Tanemura, Low temperature deposited graphene by surface wave plasma CVD as effective oxidation resistive barrier, Corros. Sci. 78 (2014) 183. [52] S. Hussain, M.W. Iqbal, J. Park, M. Ahmad, J. Singh, J. Eom, J. Jung, Physical and electrical properties of graphene grown under different hydrogen flow in low pressure chemical vapor deposition, Nanoscale Res. Lett. 9 (2014) 546. [53] N. Reckinger, A. Felten, C.N. Santos, B. Hackens, J.-F. Colomer, The influence of residual oxidizing impurities on the synthesis of graphene by atmospheric pressure chemical vapor deposition, Carbon 63 (2013) 84.

44