Controlling hydrogen environment and cooling during CVD graphene growth on nickel for improved corrosion resistance

Accepted Manuscript Controlling hydrogen environment and cooling during CVD graphene growth on nickel for improved corrosion resistance M.R. Anisur, P. Chakraborty Banerjee, Christopher D. Easton, R.K. Singh Raman PII:

S0008-6223(17)31079-5

DOI:

10.1016/j.carbon.2017.10.079

Reference:

CARBON 12506

To appear in:

Carbon

Received Date: 9 May 2017 Revised Date:

20 September 2017

Accepted Date: 23 October 2017

Please cite this article as: M.R. Anisur, P. Chakraborty Banerjee, C.D. Easton, R.K. Singh Raman, Controlling hydrogen environment and cooling during CVD graphene growth on nickel for improved corrosion resistance, Carbon (2017), doi: 10.1016/j.carbon.2017.10.079. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract

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Controlling Hydrogen Environment and Cooling during CVD Graphene Growth on Nickel for Improved Corrosion Resistance

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M.R. Anisur1, P. Chakraborty Banerjee1, 2, *, Christopher D. Easton3, R.K. Singh Raman1, 2, *

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Email: [email protected], [email protected]

Department of Mechanical & Aerospace Engineering 2 Department of Chemical Engineering, Monash University, Clayton, Victoria, Australia 3 CSIRO, Clayton, VIC 3168, Australia

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Abstract

Lack of uniformity and generation of defects including grain boundaries and wrinkles

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in graphene coatings synthesized using chemical vapour deposition (CVD) adversely affect

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the durability of these coatings. In order to control the defect density and to improve the

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durability of the resultant graphene coating, a fundamental understanding on the influence of

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the CVD parameters on the defect density is of utmost importance. In this study, the

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influences of hydrogen flow during graphene growth and the cooling rate on the defect

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density and barrier properties of a graphene coating have been investigated. A thorough

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microscopic and spectroscopic investigation revealed that (i) slow cooling hindered the

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formation of graphene coating irrespective of the presence and absence of hydrogen flow, and

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(ii) under rapid cooling condition, absence of hydrogen flow restricted wrinkle formation on

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the resultant coating. Diminished wrinkle formation in absence of hydrogen flow

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significantly improved the durability of the resultant coating. Based on an in-depth

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electrochemical impedance spectroscopic investigation, a mechanism has been proposed,

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which was further corroborated with the post corrosion analyses using X-ray photoelectron

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spectroscopy and scanning electron microscopy. This study provides a new direction to

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achieve graphene coatings with minimal defect density and excellent barrier properties.

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1. Introduction Corrosion and its mitigation annually cost ~4% of GDP of any developed economy

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(i.e., ~$250b to USA) [1]. Several approaches have been employed to mitigate the

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longstanding problem of corrosion, with varying degrees of success. In recent times,

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graphene has been identified to possess good barrier properties due to its chemical inertness

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[2] and has been reported to enhance the corrosion resistance of various metallic substrates

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[3]. Chemical vapour deposition (CVD) was one of the first techniques and is still an

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effective way to synthesize high performance pristine graphene coatings on various metal

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substrates (e.g., Cu, Ni ), which also act as catalysts for graphene growth [4-7]. However, the

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CVD graphene growth parameters, such as, the growth atmosphere (e.g., Ar/H2),

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hydrocarbon flow rate, growth temperature and post-CVD cooling rate have been reported to

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influence the generation of various defects, such as, point defects, vacancies, grain

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boundaries, wrinkles and cracks in the graphene coating [8-12].

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These defects in the CVD graphene films are deleterious for corrosion resistance, as

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they facilitate transport of oxygen and other corrosive ions to the metal surface underneath

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the coating [13-15]. Accordingly, the coating defects act as the metal oxidation initiation

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sites, which delaminate the coating with increasing exposure time in a corrosive environment

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[16]. Therefore, the defect contents in graphene films are needed to be controlled/minimized

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to achieve durable graphene coating. Even though the influence of CVD parameters on

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graphene growth has been explored extensively, few studies have reported the correlation

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between these parameters, the defect density in the resultant graphene film, and their barrier

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properties [14].

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ACCEPTED MANUSCRIPT A large number of studies had used a wide range of CVD parameters to grow

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graphene on a variety of metal substrates [1, 4-6, 13, 16-24]. A summary of these reports

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(Table S1) suggested that graphene growth temperatures are in general in the range of 800-

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1050ºC [4, 7, 13, 17], and 0-80 sccm H2 flow rate has been used along with a hydrocarbon

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source during graphene growth [4, 13, 19, 21]. The cooling condition (post graphene growth)

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varied from rapid to slow cooling in absence or presence of hydrocarbon [5, 21, 22, 25]. A

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few reports on CVD graphene growth concluded that (i) low graphene growth temperature

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[10, 26], (ii) introduction of H2 during graphene growth [27-29] and (iii) random cooling

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conditions [11, 30] may develop defects in the resultant graphene. Thus, it is reasonable to

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assume that variation in CVD parameters can result in graphene films containing different

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defect densities, which explains the inconsistencies in the performance of graphene coatings

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grown under different CVD conditions. In fact, it is evident in Table S1 that while some

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studies reported significant improvement in corrosion resistance of a metal substrate due to

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CVD graphene coatings [4, 5, 13, 17, 31], others argued that graphene coatings significantly

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accelerated corrosion of various metallic substrates [7, 18, 20]. Such variations in corrosion

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resistance due to CVD graphene coatings are attributed primarily to the large variations in the

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defect densities, irregularities, extent of surface coverage and presence of cracks in the

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graphene film [1, 16, 20, 22, 23].

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performance, understanding the influence of a few key CVD process parameters on defect

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generation and in turn on the barrier properties of graphene coating is of prime importance.

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Since the defect contents affect the graphene coating

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Generally, a reducing atmosphere consisting of Ar and H2 is maintained during the

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CVD process [32]. H2 reduces the native oxide layer on the metal substrate surface, and

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thereby facilitates graphene deposition. Prolonged annealing increases metal grain size,

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which promotes growth of large graphene domains and decreases the graphene nucleation

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density. As a result, the overall defect density in the synthesized graphene films decreases

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due to decrease in grain boundary defects [8]. Borah et al. [33] have reported the influence of

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prior annealing in presence of H2 to facilitate growth of large graphene domains with less

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defects.

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After annealing the metal surface in presence of Ar/H2, the CVD graphene growth is

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performed at high temperature with hydrocarbon flow. The high growth temperature and

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optimized hydrocarbon flow have been reported [12, 26] to facilitate growth of graphene

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films with large domains and low defect densities. Inclusion of hydrogen at a limited rate

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during graphene growth also increases the graphene domain size and growth rate [29].

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Conversely, presence of hydrogen during the hydrocarbon flow at the high temperature is

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accountable for developing wrinkle in graphene during post graphene growth cooling [27]. A

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considerable amount of hydrogen can diffuse into the metallic substrate at the high

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temperature, and subsequently diffuse out during the cooling (since hydrogen solubility

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decreases with temperature) [28]. The rejection of the dissolved hydrogen from metal matrix

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during post-CVD cooling may cause additional wrinkles (over and above, the wrinkles that

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develop due to difference in the thermal expansion co-efficient of graphene and metallic

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substrate [34]) [27]. The problem of wrinkle formation due to hydrogen may be ameliorated

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if instead of the Ar/H2 gas mixture only Ar was used along with hydrocarbon. In fact,

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hydrogen generated due to the decomposition of hydrocarbon may be sufficient to maintain

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the required reducing atmosphere for graphene growth [27, 28]. Rate of cooling after CVD is

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another parameter that has been reported to influence the uniformity of graphene coatings

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[11, 35].

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ACCEPTED MANUSCRIPT Even though there have been plethora of reports mentioning the possible influence of

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growth parameters on defect densities in graphene films, there is no comprehensive report

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providing a mechanistic insight into the role of (i) hydrogen flow during graphene growth and

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(ii) the cooling rate on the defect density and barrier properties of the graphene coatings.

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Furthermore, it is evident that little is known about the barrier properties of the graphene

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coatings over extended period of immersion in corrosive media [7]. The only long term study

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by Krishnamurthy et al. [36] reported 30 days durable coating performance of less defective

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trilayer graphene against microbial corrosion. The CVD parameters used in this study for

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graphene synthesis were used in a previous study [37] which reported presence of wrinkles

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and ripples in the developed graphene film. Another study that briefly mentioned the

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corrosion resistance of a graphene layer over 1000 h of immersion in 1500 ppm H3BO3 and

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2.3 ppm LiOH [38] did not report any significant improvement or any mechanistic insight.

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None of these studies discussed the correlation between the CVD process parameters, the

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defect density in the resultant graphene film, and their barrier properties.

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In this work, we comprehensively investigate the role of (i) hydrogen flow during

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graphene growth and (ii) the cooling rate on the defect density and barrier properties of the

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graphene coatings. The in depth understanding on the influence of these parameters led to the

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synthesis of a graphene coating with low defect density, which provided significant corrosion

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protection to the Ni substrate for 1008 h in 0.1 M NaCl. A detailed electrochemical,

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microscopic and spectroscopic analyses provided a mechanistic insight into the long term

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barrier properties of the resultant graphene coating.

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2. Experimental Procedure

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2.1 Graphene coating synthesis Ni (99.9945 Alfa Aesar no. 012043.FI) specimens (13 mm x 13 mm x 1 mm) were

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ground with silicon carbide papers up to 2500-grit finish, rinsed with acetone and deionized

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water, and subsequently dried by compressed air. A few different conditions were utilised to

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grow graphene and these conditions are summarised in Table 1. Prior to graphene growth, the

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Ni specimens were annealed for 40 min in presence of Ar/H2 (85/15 vol. %) atmosphere.

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Subsequently graphene was grown at a temperature of 1070 ºC with 1 sccm n-C6H14 (n-

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hexane) for 60 min. Graphene coating synthesized (i) in absence of hydrogen under rapid

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cooling, (ii) in presence of hydrogen under rapid cooling and (iii) in absence of hydrogen

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under slow cooling will henceforth be known as RCWOH, RCWH and SCWOH respectively.

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Post CVD graphene growth, the specimens were either rapidly cooled (RCWOH or RCWH)

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or slowly (at 0.1 º C/min) cooled (SCWOH) to room temperature under the Ar/H2 (85/15

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vol. %) environment. The schematic of the CVD set up and further details on the graphene

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growth process are illustrated in the supporting information section S2.

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Table 1 Graphene growth conditions

RCWOH (Rapid Cooling Without Hydrogen)

Graphene growth conditions Gr growth without H2 in the environment

Post graphene growth cooling conditions (in Ar/H2 (85/15 vol.%) environment) Rapid cooing

RCWH (Rapid Cooling With Hydrogen)

Gr growth with Ar/H2 (85/15 vol.%) in the environment

Rapid cooing

SCWOH (Slow Cooling Without Hydrogen)

Gr growth without H2 in the environment

Slow cooing (0.1 ºC/min)

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2.2 Scanning electron microscopy (SEM) The surface morphologies of the graphene coated and uncoated specimens before and

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after corrosion in 0.1 M NaCl were observed using JEOL JSM-7001F FEGSEM with an

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accelerating voltage of 15 kV.

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2.3 XPS analysis

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X-ray photoelectron spectroscopy (XPS) analysis was performed using an AXIS Nova

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spectrometer (Kratos Analytical Inc., Manchester, UK) with a monochromated Al Kα source

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at a power of 180 W (15 kV × 12 mA) and a hemispherical analyser operating in the fixed

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analyser transmission mode. The total pressure in the main vacuum chamber during analysis

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was typically between 10-9 and 10-8 mbar. Survey spectra were acquired at a pass energy of

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160 eV. To obtain more detailed information about chemical structure, oxidation states etc.,

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high resolution spectra were recorded from individual peaks at 40 eV pass energy (yielding a

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typical peak width for polymers of 1.0 eV).

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Each specimen was analysed at an emission angle of 0° as measured from the surface

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normal. Assuming typical values for the electron attenuation length of relevant

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photoelectrons the XPS analysis depth (from which 95 % of the detected signal originates)

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ranges between 5 and 10 nm for a flat surface. As the actual emission angle is ill-defined for

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rough surfaces (ranging from 0º to 90º), the sampling depth may range from 0 nm to approx.

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10 nm.

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Data processing was performed using CasaXPS processing software version 2.3.15

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(Casa Software Ltd., Teignmouth, UK). All elements present were identified from survey

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spectra. The atomic concentrations of the detected elements were calculated using integral

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peak intensities and the sensitivity factors supplied by the manufacturer. Binding energies

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were referenced to the C 1s peak at 285 eV for aliphatic hydrocarbon and high resolution

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spectra were normalised to peak area. The accuracy associated with quantitative XPS is ca.

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10% - 15%. Precision (i.e., reproducibility) depends on the signal/noise ratio but is usually

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much better than 5%. The latter is relevant when comparing similar samples.

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2.4 Raman spectroscopy analysis

Raman spectra of the graphene coated Ni specimens were obtained using Renishaw

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Invia Raman spectrometer equipped with 514 nm wavelength green laser (10% of laser

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power) with 1 µm spot size under a 50x objective. Multiple scans (10 scans) were performed

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to obtain the average spectra.

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2.5 Electrochemical characterization

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The corrosion behaviour of the coated and uncoated specimens (i.e., RCWOH,

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RCWH and SCWOH and uncoated Ni) were evaluated at room temperature using a Princeton

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Applied Research potentiostat (Model 2273) and a conventional three electrode

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electrochemical cell (platinum mesh was used as the counter electrode (CE), a saturated

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calomel electrode (SCE) was used as the reference electrode and the coated or uncoated

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specimens with an exposed area of 0.785 cm2 were used as the working electrode). The

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surface area of the exposed mesh counter electrode was 95 cm2 (the schematic of the cell and

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CE are shown in Figure S3 in supporting information section S3), which was much larger

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than the working electrode. The electrolyte used for the electrochemical tests was 0.1 M

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NaCl. Open Circuit Potential (OCP) was monitored for 1 h to confirm its electrochemical

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stability. A fluctuation of OCP within 10 mV for a period of 1000 s was considered as a

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stable potential before carrying out the electrochemical measurements [39, 40].

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Potendiodynamic polarization was carried out in the voltage range of – 250 mV to 250 mV

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vs. OCP at a constant voltage scan rate 0.5 mV/s. Electrochemical impedance spectroscopy

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ACCEPTED MANUSCRIPT (EIS) was performed over a frequency range of 1 MHz to 10 mHz using 10 mV perturbation

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potential. EIS was performed on the same substrate (as EIS is a non-destructive

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electrochemical technique [41]) at every 48 h interval during 1008 h of immersion in 0.1 M

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NaCl. Impedance analysis was carried out using PAR ZSimpWin package for Windows

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generally for frequencies between 1 MHz and 10 mHz to prevent misinterpretation of any

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artefacts that may be present in high frequency region, or the scatter in low frequency region.

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It must be noted here that for the potentiodynamic polarisation experiments (which is a

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destructive technique), two different cells were used to gather data at 2 h and at 1008 h

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respectively. In order to examine the reproducibility of the results, all the electrochemical

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experiments were repeated at-least thrice.

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3. Results and discussion

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3.1 Morphology and chemical characterization of the graphene coating Figure 1a shows that the morphology of the SCWOH specimen consists of Ni

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grains and grain boundaries (a typical grain identified by the white dashed line) with

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possible presence of Ni-C compounds [42]. However, absence of graphene domain in this

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specimen suggests that under slow cooling condition, graphene growth is inhibited and

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instead Ni-C compounds form. Absence of G (~1582 cm-1) and 2D (~2670 cm-1) peaks in

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the Raman spectra of this specimen (Figure 1d) further confirms the absence of any

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graphene coating in case of the SCWOH specimen. This observation is not surprising if

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one considers the fact that carbon solubility in Ni is temperature dependent [35, 42, 43].

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The carbon diffusivity, DT at temperature T ºK into bulk Ni can be estimated by

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[43]



 =  exp (−  ) (in cm2s-1),

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factor, Boltzmann’s constant and the diffusion activation energy respectively. This

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relationship gives the diffusion length, & = 2' ( (where t is the diffusion time) of 1 mm

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at 1070 ºC for 1 h of carbon precursor flow. On this basis, the Ni foils in the present study

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being 1 mm thick can be considered to be saturated with C atoms after 1 h of carbon

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precursor flow. In the slow cooling scenario, the extended cooling for 2 days (Figure S1c)

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provided enough opportunity for carbon to form Ni-C compounds (Figure 1a). Thus it can

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be presumed that excessive carbon diffusion during slow cooling was not conducive for

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graphene coating formation.

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Unlike the slow cooling condition, graphene growth was observed under the rapid

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cooling conditions (Figures 1b-c). In fact, the IG/I2D ratio of 1.12 and 1.11 in case of the

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RCWH and RCWOH specimens respectively suggest presence of 3-4 layers of graphene in

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these specimens (Figure 1d) [44]. Figures 1b-c show that the graphene coating RCWH

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specimen (Figure 1b) has relatively more wrinkles (shown by arrows in Figure 1b) than

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that of the RCWOH specimen (Figure 1c), suggesting that presence of H2 facilitates

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wrinkle formation on the resultant graphene coating. Furthermore, the acquired images

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(Figure 1b-c) were then analysed with Fiji image analysis software (NIH, US) to evaluate

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area percentage of visible wrinkles from the images. The area percentage from binary image

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(8 bit) in Figure S4 in supporting information section S4 was determined with the ‘measure’

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function. The analyses demonstrate that higher wrinkle density (17%) (Figure S4c) is

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present in the graphene film developed through RCWH condition due to inclusion of

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hydrogen than that (9%) (Figure S4d) in the one developed by the RCWOH condition.

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Additionally, the high magnification of the D peak region (1300-1400 cm-1), which

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represents the defect density in a graphene coating [45], shows higher intensity of D peak

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ACCEPTED MANUSCRIPT in RCWH than in the case of RCWOH (inset of Figure 1d), suggesting generation of

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relatively higher defect density in presence of H2. Raman spectral mapping of 20 x 20 µm2

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was performed for further ID/IG (defect intensity) and IG/I2D (number of graphene layer) ratio

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analyses of graphene coating developed by RCWOH and RCWH conditions (Figure S5 in

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supporting information section S5). This mapping of ID/IG confirms that H2 flow during

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graphene growth affects the overall quality of graphene, as higher defect intensity ratio is

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found for the RCWH (Figure S5b) than that of RCWOH (Figure S5a). This is in agreement

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with the literature [27], and agrees well with the observation of higher density of wrinkles

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visible in the case of the RCWH specimen (Figure 1b). However, it is noticeable that the

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IG/I2D ratio of RCWOH (Figure S5c) and RCWH (Figure S5d) also confirm the complete

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coverage of graphene.

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The graphene coated and uncoated Ni specimens were further examined by XPS.

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Atomic concentrations derived from the survey spectra of RCWOH, RCWH and SCWOH are

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presented in Table S2 (supporting information section S6), while the survey spectra are

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presented in the supporting information section S7. In agreement with the SEM observation

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and Raman analysis, carbon (C) was present in large quantities in case of the RCWH and

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RCWOH specimens, confirming the presence of graphene coatings in these specimens. The

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high resolution C 1s spectra for RCWOH, RCWH and SCWOH are shown in Figure 2. The

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binding energy position of the main peak (284.4 eV) and the overall spectral shape, including

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the loss structure at high binding energy, for RCWH and RCWOH are indicative of graphitic

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carbon [46-48]. The principal C peak at 285 eV in case of the SCWOH sample (Figure 2) is

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indicative of aliphatic hydrocarbon while another peak at ~288.6 eV is associated with

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acid/ester groups [49]. Thus, the C peak detected for this sample may have arisen because of

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hydrocarbon contamination. Additionally, presence of large quantities of Ni (Table S2 in the

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supporting information S6) and no loss structure at high binding energy (Figure 2) indicate

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confirms that slow cooling does not allow synthesis of graphene coating. A small amount of

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Ni (0.26%) (Table S2 in the supporting information S6) was also detected in the case of the

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RCWH specimen indicating possible presence of discontinuities/cracks in this coating.

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Presence of cracks in RCWH is not surprising if one considers that this specimen was

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synthesized in presence of H2 that facilitates wrinkle formation, and in general, cracks

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develop along these wrinkles due to bending of graphene films which induce a uniaxial strain

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perpendicular to the wrinkles [22].

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3.2 Electrochemical measurements

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In this section, we have investigated the barrier properties of the graphene coated

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specimens at various durations of immersion in 0.1 M NaCl, and compared their corrosion

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resistances with the uncoated Ni substrate. However, since our SEM, Raman spectra and XPS

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analysis confirmed that slow cooling did not allow graphene growth, we have not considered

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the electrochemical response of the SCWOH specimen here. However, the corrosion

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performance of SCWOH has been reported in supporting information S8.

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3.2.1 Coating performance of RCWOH and RCWH

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Figure 3a shows the potentiodynamic polarization plots of the graphene coated Ni

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(RCWOH and RCWH) and the uncoated Ni specimens after 2 h of immersion in 0.1 M NaCl.

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The corrosion potential (Ecorr), which is a measure of corrosion susceptibility [40, 50, 51],

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was similar in case of the RCWH and the uncoated Ni specimens, whereas Ecorr of RCWOH

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was 53 mV more positive suggesting RCWOH to be less susceptible to corrosion after 2 h of

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immersion. The anodic and cathodic current densities of the coated specimens were lower

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than those for the uncoated Ni, suggesting that both the coatings provided protection to the Ni

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substrate underneath. However, the anodic current density (which is a measure of the metal

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lower than that of the uncoated Ni specimen until an over potential of 0.05 V vs SCE.

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Beyond this over potential, the anodic current densities of both the RCWH and RCWOH

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specimens increased steadily indicating degradation of the graphene coating at these over

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potentials. The similar anodic current density (sweeps towards positive side) for both

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RCWOH and RCWH at the higher over potentials (> 0.05 V vs SCE) can be attributed to the

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damage of the graphene coatings under high over potentials. However, at over potentials

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below 0.05 V, the anodic current density of the RCWOH specimen was lower than that of the

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RCWH specimen, due to the suggested role of the lower defect density in the graphene

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coatings in improving corrosion resistance of RCWOH. Damage in the highly cathodic

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graphene coatings can create localized galvanic cells accelerating the anodic dissolution rate

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of the Ni substrate underneath [4].

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Since the RCWOH specimen showed the lowest anodic and cathodic current densities

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at 2 h of immersion in 0.1 M NaCl (Figure 3a), the long term durability of this graphene

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coating was examined by performing potentiodynamic polarization after pre-immersing this

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specimen in 0.1 M NaCl for 1008 h (Figure 3b). The cathodic current density of the RCWOH

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specimen (Figure 3b) immersed for 1008 h was similar to that immersed for 2 h in 0.1 M

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NaCl, and was at least an order of magnitude lower than that of the uncoated specimen. The

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anodic current density of RCWOH after 1008 h was higher than that at 2 h of immersion, but

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was lower than the uncoated Ni specimen, suggesting that even though the coating may have

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gradually degraded over time, it still provided significant corrosion protection to the Ni

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substrate. It is interesting to note here that the graphene coating on the RCWOH specimen

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immersed in 0.1 M NaCl for 1008 h was more stable at a broader over potential range (Ecorr -

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0.18 V vs SCE) than the specimen at 2 h of immersion. Similar phenomenon were observed

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coatings with corrosion products over prolonged immersion. However, beyond 0.15 V vs

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SCE, the increase in the anodic current density presumably be attributed to coating

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degradation at high over potentials facilitating galvanic coupling between the cathodic

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graphene coating and the Ni substrate.

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Figure 4a shows the BODE modulus plots of the RCWOH, RCWH and the uncoated

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Ni specimens at 2 h of immersion in 0.1 M NaCl. Modulus of impedance (|Z|) at the lowest

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frequency (10 mHz in this study) is a measure of the corrosion resistance of a system [41]. In

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agreement with the potentiodynamic polarization results (Figure 3), the highest corrosion

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resistance was achieved in case of the RCWOH specimen at 2 h of immersion (Figure 4a).

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Figure 4b demonstrates that the corrosion resistance of RCWOH was higher than that of the

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uncoated Ni specimen even after 1008 h of immersion in 0.1 M NaCl. However, after 1008 h

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the corrosion resistance of the RCWOH specimen was lower than that at 2 h of immersion,

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which can be attributed to the gradual degradation of the graphene coating over time.

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In order to investigate the dissolution kinetics of the graphene coated (RCWOH and

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RCWH) and uncoated Ni specimens, the corrosion resistances (corresponding BODE plots

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are shown in the supporting information section S9) of these specimens were monitored at

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different durations of immersion (Figure 5a). The corrosion resistance of the RCWOH

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specimen decreased in the first 48 h and then remained constant until 1008 h of immersion.

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The corrosion resistance of this specimen even at 1008 h of immersion was at least an order

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of magnitude higher than that of the uncoated Ni specimen. The corrosion resistance of the

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RCWH specimen was nearly an order of magnitude higher than that of the uncoated Ni

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was similar to the uncoated Ni specimens for the rest of the durations of immersion in 0.1 M

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NaCl. The considerably higher corrosion resistance of the RCWOH than that of the RCWH

341

(Figures 3, 4 and 5a) can be attributed to the absence of hydrogen during the CVD synthesis

342

in case of the RCWOH specimens. In the absence of hydrogen, development of wrinkles is

343

ameliorated. Wrinkles are well known to facilitate diffusion of oxygen and other corrosive

344

ions through the graphene coating [13, 16, 22].

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Time constants in Bode phase angle plots represent various electrochemical processes

347

in an electrochemical system. The nature and specifics of the electrochemical processes can

348

be determined by noting the characteristic frequency at which the maximum of a particular

349

time constant occurs [53]. In general for a coated system undergoing corrosion, the high

350

frequency time constant represents the charge transfer processes at the coating/electrolyte

351

interface, the medium and low frequency time constants represent the hydroxide layer/metal

352

electrode/electrolyte interfaces [51, 54, 55]. At times, when the maxima of two or more time

353

constants occur at characteristic frequencies close to each other, these time constants overlap,

354

and result in a broader time constant [55]. The characteristic frequency of the maximum of

355

the broad time constant depends on the predominance of the response of a particular interface

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[53].

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Graphene coatings have been reported [4] to show a broad time constant, which is

359

generally attributed to the overlap of the two or more time constants related to the responses

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at the graphene coating/electrolyte and metal substrate/metal hydroxide/electrolyte interfaces

361

[4, 6]. The phase angle plots (Figures 5b and c) of both the graphene coated specimens 15

ACCEPTED MANUSCRIPT (RCWOH and RCWH) showed a broad time constant at 2 h of immersion in 0.1 M NaCl,

363

which is in agreement with the literature [4] and can be attributed to the overlap of the time

364

constants representing responses of various interfaces. The distinct high frequency and low

365

frequency time constants after 48 h in case of the RCWOH specimen (Figure 5b) can be

366

attributed to the coating/electrolyte and metal/electrolyte interfaces [51, 54, 55]. Since the

367

response of a protective coating/electrolyte interface occur at high frequencies [53, 55], and

368

the diffusion and charge transfer processes at the metal/electrolyte interface occur at low

369

frequencies, the two time constants do not overlap, and hence, we find them more prominent

370

in the phase angle plot. Additionally, the high frequency time constant corresponds to a phase

371

angle of ~ 80˚ at all durations of immersion confirming the capacitive nature of this coating,

372

which further explains the high corrosion resistance achieved in the case of RCWOH at all

373

durations of immersion.

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In case of the RCWH coating, the high frequency time constant shifted towards the

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medium-low frequency range with increasing immersion time. In fact, after 48 h the partially

377

overlapped broad time constant appeared at the same frequency range as that of the uncoated

378

Ni, suggesting the less protective nature of this coating and the onset of electrochemical

379

processes similar to the uncoated metal surface. This further confirms the exposure of the

380

metal substrate due to coating delamination and can be attributed to higher wrinkle density in

381

RCWH than that of RCWOH as it is observed from the image analysis of the graphene coated

382

specimens (Figure S4 in supporting information section S4). Hence, it is evident that the

383

presence of higher wrinkle density adversely affects the barrier properties of graphene.

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16

ACCEPTED MANUSCRIPT To obtain a more mechanistic insight into the presumable effect of hydrogen induced

386

wrinkles on the time dependent evolution of the resultant graphene coating/electrolyte and the

387

Ni substrate/electrolyte interfaces, and their influence on the overall corrosion processes for

388

the RCWOH and RCWH specimens, a detailed EIS analysis was performed using an

389

electrical equivalent circuit (EEC). In the present study, complex nonlinear least squares

390

(CNLS) method was used. The fitting procedure, circuit description code (CDC) and the

391

weighing modulus are described elsewhere [51].

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Figure 6a shows the proposed EEC. Briefly, in this EEC, Rs is the electrolyte

394

resistance, the graphene coating/electrolyte interface is represented by a constant phase

395

element (CPE), QGr, and a pore resistance, RGr, while the Ni substrate/electrolyte interface

396

is represented by another CPE, Qdl, and a resistance Rdl. Incorporation of CPEs in the

397

proposed EEC improved the agreement between the simulated and the experimental

398

impedance data, which is justified by noting the distributed surface reactivity, roughness,

399

electrode porosity, current and potential distributions associated with the electrode geometry

400

[4]. The simulated data with our proposed EEC were in good agreement with the

401

experimental data as shown in Figure 6b (the detailed data validation is shown in supporting

402

information section S10). The error plots (Figure S11 in supporting information S11) show

403

that the maximum error in |Z| calculation was less than 2% and the error in the phase angles

404

were less than ± 1o. The time dependent evolution of the graphene coating/electrolyte

405

interface and the Ni substrate/electrolyte interface is shown in Figure 7 (the error bars from

406

the triplicate tests are shown in Figure S12 in supporting information section S12). The

407

coating capacitance, QGr (Figure 7a), increased with time for both the coated specimens,

408

suggesting increase in the number of conductive pathways through the graphene coatings

409

[55, 56]. This is further confirmed by the decrease in the pore resistance, RGr (Figure 7b),

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ACCEPTED MANUSCRIPT with time. The capacitive response of any interface is proportional to the area of exposure

411

[57], and hence, an increase in Qdl corresponds to an increase in the exposed area of the Ni

412

substrate/electrolyte interface. The rapid increase in Qdl (Figure 7c) of the graphene coated

413

specimens until 300 h can be correlated with the rapid decrease in the pore resistance (RGr)

414

of the graphene coating, which facilitated facile electrolyte access to the Ni substrate

415

underneath the coating. Lower QGr and higher RGr in case of the RCWOH specimen

416

confirm development of a highly resistive graphene coating with lower density of

417

conductive pathways in the absence of hydrogen during the CVD process. Highly resistive

418

graphene coating in the case of the RCWOH specimen explains the limited exposure and

419

high resistance (Rdl) (Figure 7d) at the Ni substrate/electrolyte interface at all times of

420

duration in 0.1 M NaCl. This further explains the enhanced long term corrosion resistance

421

achieved in the case of the RCWOH specimen (Figures 3, 4 and 5a) and proves our

422

hypothesis that absence of hydrogen will impede wrinkle formation on the resultant

423

graphene coating and thereby would provide long term corrosion protection to the metal

424

substrate underneath.

427 428

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3.3 Post corrosion morphology, Raman mapping and XPS analysis

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The post corrosion morphology of the graphene coated and uncoated specimens are

429

shown in Figure 8. Uncoated Ni specimens immersed in 0.1 M NaCl for just 96 h suffered

430

severe corrosion (localized) damages over entire surface in the form of large pits (Figure 8a,

431

b). A considerable amount of pitting is seen also in case of the SCWOH specimen after 96 h

432

immersion (Figure 8c) which is clearly seen at a higher magnification (Figure 8d). A typical

433

grain boundary of this sample is identified by the rectangular enclosure with dotted lines in

434

Figure 8c. The RCWH specimen immersed in 0.1 M NaCl for 1008 h suffered corrosion 18

ACCEPTED MANUSCRIPT damages and coating disruptions (identified by the rectangular enclosures with dotted lines)

436

especially in the areas with wrinkles (Figure 8e, f). However, only a few minor

437

discontinuities / disruptions (identified by dotted lines) were observed in case of the RCWOH

438

specimen after 0.1 M NaCl for 1008 h (Figure 8g, h). Consistent with the qualitative features

439

of the disruption seen in Figure 8e-8h, the area of graphene delamination was calculated to be

440

5% in case of the RCWH while it is only 1% for the RCWOH (Figure S13 in supporting

441

information S13). Raman spectral mapping data confirm the inferences out of SEM results,

442

i.e., after exposure to 0.1 N NaCl for 1008h, the graphene coating remained relatively intact

443

on RCWOH (Figure 8g and 8h) but suffered disruptions/delamination in the case of the

444

RCWH (Figure 8e and 8f).

445

maps provide a qualitative comparison of defect contents of RCWOH and RCWH.

446

expected, the IG/I2D maps for the unexposed samples confirm presence of graphene on

447

RCWOH (Figure S5c) as well as RCWH (Figure S5d). However, the ID/IG maps for the

448

unexposed samples suggest a greater degree of defects in the case of RCWH (Figure S5b)

449

than for RCWOH (Figure S5a). The IG/I2D maps for the two samples after 1008 h exposure

450

suggest most of the surface of RCWOH to be still covered with graphene (Figure S5g)

451

whereas graphene was present only in isolated areas in the case of the RCWH (Figure

452

S5h). As expected, the ID/IG maps after 1008 h exposure suggest a greater degree of the

453

inherent defects in RCWH (Figure S5f) than for RCWOH (Figure S5e). As discussed earlier,

454

the greater content of certain defect types (domain boundaries/wrinkles) would facilitate

455

greater corrosion rate (as in the case of RCWH). Further, presence of Ni in the XPS survey

456

spectra of the RCWOH specimen was negligible, whereas, an increased concentration of Ni

457

(10.5%) was observed in case of the RCWH specimen (Table S2 in supporting information

458

section S6). Results of the post corrosion characterisation of morphology of RCWOH and

459

RCWH and surface analyses by XPS and Raman spectroscopy agree well with the

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As

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The IG/I2D maps confirm the presence of graphene whereas ID/IG

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electrochemical data and confirm that defects in graphene coatings (wrinkles/domain

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boundaries) act as sites for corrosion initiation and propagation. This inferences emphasizes

462

the importance of the development of strategies to eliminate wrinkles from graphene

463

coatings.

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4. Conclusion

This paper presents the effect of CVD graphene growth parameters on graphene

467

coating on a Ni substrate and its effect on the durability of corrosion resistance of Ni. The

468

current study demonstrates the effect of wrinkle formation as a result of the presence of H2 in

469

the inert environment during graphene growth in the quality of graphene coating and its

470

significant durable corrosion resistance in 0.1 M NaCl. The potentiodynamic and EIS tests

471

show that graphene coatings developed on Ni under the environment with or without H2 offer

472

nearly one and half order of magnitude improvement of corrosion resistance than that of

473

uncoated Ni in 2 h of immersion in 0.1 M NaCl. However, the graphene coating developed in

474

the presence of H2 showed nearly same corrosion resistance as uncoated Ni upon further

475

exposure/immersion for 48 h, whereas the CVD graphene coating developed in the absence

476

of H2 consistently showed at least an order of magnitude higher corrosion resistance than that

477

of uncoated Ni for the entire immersion period of 1008h. The effect of cooling rate after

478

CVD for graphene growth was also investigated. However, graphene growth was not

479

successful for slow cooling condition, and the samples generated upon this treatment did not

480

provide any significant improvement in corrosion resistance.

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Acknowledgement

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The authors would like to thank department of mechanical and aerospace engineering,

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monash university, MIGR and MCATM for providing the required facilities and funding for

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ACCEPTED MANUSCRIPT the project. The authors would like to thank Monash centre for electron microscopy (MCEM)

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for supporting this work with microscopy facilities.

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References

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Ni grain boundary with possible Ni-C compound

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Raman shift (cm-1)

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Fig. 1 SEM and Raman spectroscopy to examine graphene coating quality on Ni. (a-c) SEM images show no trace of graphene film/wrinkles on SCWOH (a), and distinguishable wrinkles on RCWH (b) and RCWOH (c), and d) Raman spectra with graphene characteristic peaks (G and 2D) absent in SCWOH, and present in RCWH and RCWOH. The inset shows the Raman spectra from wavenumber 1150 to 1450 cm-1 which identify the D peak for RCWH only (but absence of D peak for SCWOH and RCWOH)

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Normalised intensity (arb. units)

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Fig. 2 Selected, representative high resolution C 1s spectral overlay from samples SCWOH, RCWH and RCWOH. Insert presents the same data, with axes range reduced to highlight spectral features at high binding energy.

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-5

-4

-3

log i (A/cm2)

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b)

0.2

Potential (V) vs SCE

Potential (V) vs SCE

a) 0.3

-6

-5

-4

-3

log i (A/cm2)

AC C

EP

TE D

M AN U

SC

Fig. 3 Potentiodynamic polarization plots of a) RCWOH, RCWH and uncoated Ni after 2 h immersion and b) RCWOH after 2 h and 1008 h immersion and uncoated Ni after 2 h immersion

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4 3 2 1 -2

-1

0

1

2

3

RCWOH at 2 h RCWOH at 1008 h Uncoated Ni at 2 h

5

log |Z| (Ω cm 2 )

5

log |Z| (Ω cm2)

b) 6

RCWOH at 2 h RCWH at 2 h Uncoated Ni at 2 h

4

4 3 2 1 -2

-1

0

RI PT

a)

1

2

3

4

log f (Hz)

log f (Hz)

AC C

EP

TE D

M AN U

SC

Fig. 4 BODE plots of a) RCWOH, RCWH and uncoated Ni after 2 h immersion and b) RCWOH after 2 h and 1008 h immersion and uncoated Ni after 2 h immersion

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ACCEPTED MANUSCRIPT b) 90

800 600 400 200 0

80 70 60 50

2h 48 h 96 h 288 h 576 h 768 h 1008 h

40 30 20 10 0

0

200

400

600

800

1000

1200

-2

-1

80

70

70

-Phase angle (degrees)

80

60 50 2h 48 h 96 h 288 h 576 h

30 20

768 h

10

1008 h

0 -2

-1

0

1

1

2

3

4

3

4

SC

90

60

M AN U

- Phase angle (degrees)

d)

90

40

0

log f (Hz)

Immersion time (h)

c) )

RI PT

RCWOH RCWH Uncoated Ni

1000

|Z| kΩ cm2

- Phase angle (degrees)

a) 1200

2h 48 h 96 h 288 h 576 h 768 h 1008 h

50 40 30 20 10 0

2

4

TE D

log f (Hz)

3

-2

-1

0

1

2

log f (Hz)

AC C

EP

Fig. 5 a) Modulus of impedance (|Z|) at the lowest frequency with respect to immersion time for RCWOH, RCWH, uncoated Ni. Phase angle plots of b) RCWOH, c) RCWH, and d) uncoated Ni immersed in 0.1 M NaCl for 1008 h

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ACCEPTED MANUSCRIPT b)

6

90 80

5

log |Z| (Ω cm2 )

70 60

4

50 40

3 2

RI PT

30 20

Experimental Simulated data

1 -2

-1

0

1

log f (Hz)

2

3

-Phase angle (degree)

a)

10 0 4

AC C

EP

TE D

M AN U

SC

Fig. 6 a) Equivalent circuit for graphene coated Ni, b) A typical Bode plot for calculated and experimental EIS data of RCWOH

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b) 70

-4.0 RCWOH RCWH Uncoated Ni

-4.4

RCWOH RCWH Uncoated Ni

60

-4.6 -4.8 -5.0 -5.2 -5.4

50 40 30 20 10 0

-5.6 -5.8 0

200

400

600

800

1000

1200

0

-4.2

d)1200

-4.4

1000

RCWOH RCWH Uncoated Ni

-5.0 -5.2 -5.4

Rdl (kΩ cm2)

-4.6 -4.8

400

600

800

1000

1200

Immersion time (h)

800 600

Gr/Ni (RCWOH) Gr/Ni (RCWH) Bare Ni

M AN U

log Qdl (Fcm-2)

c)

200

SC

Immersion time (h)

RI PT

log QGr (Fcm-2)

-4.2

RGr (kΩ cm2)

a)

400 200

0

-5.6 -5.8 0

200

400

600

800

0

200

400

600

800

1000

1200

Immersion time (h)

TE D

Immersion time (h)

1000 1200

AC C

EP

Fig. 7 Time dependence of (a) RGr (b) Rdl (c) QGr and (d) Qdl for RCWOH, RCWH and uncoated Ni exposed to 0.1 M NaCl. The metal/coating/electrolyte interface for RCWOH and RCWH is assumed as Nickel/Graphene/electrolyte and for uncoated Nickel, Nickel/Nickel hydroxide/electrolyte is considered here.

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c)

e)

b)

d)

f)

g)

SC

RI PT

a)

M AN U

h)

AC C

EP

TE D

Fig. 8 SEM morphologies of uncoated Ni (a,b) and SCWOH (c,d) samples after 96 h; RCWH (e,f) and RCWOH (g,h) samples after 1008 h immersion in 0.1 M NaCl. The calibration bar in the figures (a,c,e,g) and (b,d,f,h) are 10 and 1 µm respectively. The images in b,d,f,h are the high magnification images of the solid lined boxed areas in a,c,e,g respectively. The areas under dotted enclosure (irregular/ circular/rectangular) correspond to the regions which indicate localized corrosion/pitting (b,d), grain boundary (c), coating delamination (e,f), coating delamination/cracks in coating (g,h).

8

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

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