Thin Solid Films 585 (2015) 31–39

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Thin Solid Films

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Surface and interfacial interactions of multilayer graphitic structureswith local environment

R. Mazzocco a, B.J. Robinson a,⁎, C. Rabot b, A. Delamoreanu c, A. Zenasni b, J.W. Dickinson d,C. Boxall d, O.V. Kolosov a

a Department of Physics, Lancaster University, Lancaster LA1 4YB, UKb CEA-LETI-Minatec Campus, 17 rue des Martyrs, 38054 Grenoble Cedex 09, Francec Microelectronics Technology Laboratory (LTM), Joseph Fourier University, French National Research Center (CNRS), 17 Avenue des Martyrs, 38054 Grenoble Cedex 9, Franced Department of Engineering, Lancaster University, Lancaster LA1 4YR, UK

⁎ Corresponding author. Tel.: +44 1524 593218; fax: +E-mail address: b.j.robinson@lancaster.ac.uk (B.J. Robi

http://dx.doi.org/10.1016/j.tsf.2015.04.0160040-6090/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 16 June 2014Received in revised form 1 April 2015Accepted 7 April 2015Available online 15 April 2015

Keywords:GrapheneThickness measurementSegregationChemical reductionAtomic force microscopyUltrasonic force microscopy

In order to exploit the potential of graphene in next-generation devices, such as supercapacitors, rechargeablebatteries, displays and ultrathin sensors, it is crucial to understand the solvent interactions with the graphenesurface and interlayers, especially where the latter may be in competition with the former, in themedium of ap-plication deployment. In this report, we combine quartz crystalmicrobalance (QCM) and ultrasonic forcemicros-copymethods to investigate the changes in the film–substrate and film–environment interfaces of graphene andgraphene oxide films, produced by diverse scalable routes, in both polar (deionised water) and non-polar(dodecane) liquid and vapour environments. In polar liquid environments, we observe nanobubble adsorp-tion/desorption on the graphene film corresponding to a surface coverage of up to 20%. As no comparable behav-iour is observed for non-polar environment, we conclude that nanobubble formation is directly due to thehydrophobic nature of graphenewith direct consequences for electrode structures immersed in electrolyte solu-tions. The amount of water adsorbed by the graphene films was found to vary considerably from 0.012 mono-layers of water per monolayer of reduced graphene oxide to 0.231 monolayers of water per monolayer ofcarbon diffusion growth graphene. This is supported by direct nanomechanical mapping of the films immersedin water where an increased variation of local stiffness suggests water propagation within the film and/or be-tween the film and substrate. Transferred film thickness calculations performed for QCM, atomic force microsco-py topography and optical transmission measurements, returns results an order of magnitude larger (46 ± 1layers) than Raman spectroscopy (1 - 2 graphene layers) on pristine pre-transferred films due to contaminationduring transfer and possible turbostratic structures of large areas.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Graphene has attracted increasing attention due to its outstandingmechanical, electrical and thermal properties [1]with significant poten-tial for the development of next-generation materials [2,3] and elec-tronic devices [4,5]. However, to fully realise its application to energystorage [6,7] and high precision sensors [8] requires detailed under-standing of graphene's interaction with its local application-specific en-vironment such as electrolytes, water and organic liquids [9–11]. Herewe explore the graphene–liquid and graphene–vapour interface for aseries of graphene films made by different and potentially industriallyscalable routes.Mechanical cleavage, which has been shown to producethe highest quality graphene [12], is not commercially viable, howeveralternative methods such as chemical vapour deposition (CVD), chemi-cal reduction of graphene oxide and ultrasonic and electrochemical

44 1524 844037.nson).

exfoliation [12–14] show considerable potential. Here, we studygraphene films produced by two diverse routes—a modified Hummersmethod that uses chemical reduction of graphene from grapheneoxide (GO) to produce reduced graphene oxide (rGO), and carbonsegregation [15], also referred to as “carbon diffusion growth” (CDG)approach [16,17].

In this paper we combine mesoscale and nanoscale measure-ments of GO, rGO and CDG using quartz crystal microbalance(QCM), atomic force microscopy (AFM) and nanomechanical map-ping via ultrasonic force microscopy (UFM), to investigate the nano-structure interfacial interactions with polar (water) and non-polar(dodecane) liquids and vapour. QCM is a highly sensitive techniquethat allows the study of physical interfaces by probing average loadmass changes down to the nanogram over a surface area of a fewcm2 [18,19]. Here it is used to determine the area density and vapourabsorption of the graphene films. These are complemented by AFMsurface morphology studies [20] and, crucially, UFM probing of theelastic properties of the surface and subsurface features including

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Fig. 1. Apparatus employed for the QCM experiments—the QCM is in good thermal contact with the Peltier heater. The desiccator lid coupled with an aluminium plate through an insu-lating gasket ensures a stable vacuum in the chamber.

32 R. Mazzocco et al. / Thin Solid Films 585 (2015) 31–39

the graphene layers and their interaction with the substrate [21–23].The combination of QCM quantitative sensitivity with the spatial res-olution of AFM and UFM allowed us to investigate the interaction ofgraphene and GO with the environment and link these with the 3Dmorphology to elucidate the properties of graphene made by diversescalable methods.

Fig. 2.AFM topographical mapping in ambient environment of (a) GO, (c) rGO and (e) CDG grapmechanical maps of GO, rGO and CDG respectively. The scans shown are of complete graphitic

2. Materials and methods

2.1. Graphene and graphene oxide via modified Hummer method

Graphene and graphene oxide were produced via the well-knownmodified Hummer method [24–26], with the following parameter:

hene transferred to the QCM surface; (b), (d) and (f) simultaneously captured UFM nano-films.

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i) crystals were rinsed in 3:1 H2SO4/H2O2 for 2 s, then rinsed in E-Pure(doubly distilled) water and blow-dried in Nitrogen; ii) crystals wereplaced into a holder exposing one Au face, which was treated with20 mM cystamine in toluene solution and left to soak for 15 min; iii)samples were then rinsed in fresh toluene, sonicated for 10 s at 20 Wpower and subsequently dried under nitrogen stream; iv) 20 μl of12.66 ± 0.23 mg/ml GO solution was spread over the treated crystalsurface and left to stand for 1 min prior to spin coating for 1 min at300 rpm/600 rpm/900 rpm/1500 rpm; v) crystals were inserted into anitrogen filled fuming bottle containing 1 ml H2N2 (98%) and thenplaced into an oven at 313 K for 20 h; vi) thermal reduction of GO torGO was carried out into a nitrogen filled furnace where a nitrogenflow rate of 5 l/min was maintained throughout the entire reductionprocess; the furnace was ramped at 300 K/h up to 400 K and thenheld at 400 K for 2 h prior to cooling and removal.

2.2. CDG graphene growth and transfer

Detailed descriptions of graphene synthesis by carbon segregationtechnique have been previously reported [16]. Graphenewas synthetisedon 8-inches p-type siliconwafer (100)with aNi(20–300nm)/a:SiCH(20–200 nm)/SiO2(300 nm)/Si multi-stack annealed at 700 °C for 3 min. Via

Fig. 3.AFM topographical mapping inDIwater environment of (a) GO, (c) rGO and (e) CDG grapmechanical maps of GO, rGO and CDG respectively. GO and rGO scans are of areas comprising

diffusion, carbon atoms migrate from the amorphous silicon carbidelayer through the nickel layer due to its low solubility when heated.After annealing, a thin layer of graphene appears on top of the nickel.The subsequent transfer on a quartz crystals involved the followingsteps: i) a 800 nm layer of poly(methyl methacrylate) (PMMA) wasspin coated on the Graphene/Ni/a:SiCH/SiO2/Si wafer, then the waferwas dipped in 27% FeCl3 and left in such a solution for a few hours toachieve complete removal of the underlying nickel substrate; ii) once allnickel had been etched, the resulting graphene/PMMA layer was firstrinsed several times in deionised (DI) water and then fished out by im-merging the desired substrate, i.e. a QCM crystal, in the liquid and subse-quently lifting it up so as to obtain the deposition of such a film on thecrystal top gold electrode; iii) ultimately, after annealing the sample at180 °C and removing the PMMA in acetone, the graphene was standingdirectly onto the crystals. In this work we referred to graphene made bythis carbon diffusion method as CDG.

2.3. QCM setup: hardware and software description

The QCM experimental setup consisted of a phase locked oscillator(Maxtek PLO 10i) providing the resonant frequency and a dc voltage pro-portional to the crystal's conductance, a frequency counter to measure

hene transferred to the QCM surface; (b), (d) and (f) simultaneously capturedUFMnano-complete graphic film coverage, CDG scans are of the Au/CDG interface.

Fig. 4. (a) Typical AFM image of the CDG graphene/Au coatedQCM substrate interface and(b) associated profiles.

34 R. Mazzocco et al. / Thin Solid Films 585 (2015) 31–39

frequency shifts (HP 53131A) and a multimeter (Keithley 6430) to mea-sure thedc signal. Frequency andvoltagewere recorded at a rate of 1 sam-ple/s. A large area Peltier heaterwas employed to control the temperaturewithin a range relevant to industrial applications; an in-house machinedaluminium crystal holder and desiccator lid enabled rapid thermal andpressure adjustments the experimental setup is shown schematically inFig. 1.

In a QCM setup, a thin crystal (usually, AT-cut quartz) is sandwichedbetween two metal electrodes that induces shear vibrational motion ofthe crystal when an AC voltage is applied. The dedicated electronic circuitwith feedback enables operation of QCM at the frequency of the thicknessresonance of the crystal [27] with any additional layer deposited on thecrystal causing a drop in frequency proportional to the thickness of thelayer [28]. In order to account for the temperature induced changes inthe frequency, in our setup all QCM crystals were first characterised fortemperature-induced frequency changes [29], by varying the tempera-ture in the range of 20–60 °C in steps of 10 °C. A set of Au crystals fromthe same batch exhibited changes in frequency ranging from −4.2 ±0.2 Hz K−1. The frequency reduction with the temperature increase oc-curs due to thermal expansion of the crystal. It is interesting to note,that for graphene coated crystals, such a change was found to be slightlylower in absolute value, being −3.56 ± 0.5 Hz K−1 for graphene oxide(GO), −3.16 ± 0.2 Hz K−1 for rGO, and −3 ± 0.2 Hz K−1 for CDG.Such changes could be explained by the fact that graphene shrinks withthe temperature increase [30] that may also create additional compres-sive strain on the crystal, graphene's coefficients of thermal expansion(CTE) is CTEGr=−8 × 10−6 K−1 [31]. It is also possible that temperaturevariations may induce decoupling of part of the flakes from the underly-ing gold substrate (CTEAu = 14 × 10−6 K−1) [32].

2.4. Vapour detection experiments

In these experiments, temperature was maintained at 60 °C for bothwater and dodecane. The crystal was allowed to reach equilibrium priorto a disc of filter paper, arranged into the aluminium holder, was sprin-kled with 0.05 ml of liquid, and allowed to equilibrate (variation main-tained within 2% of the average). The dynamics of the injection andequilibration process were.

2.5. Underliquid QCM measurements

For underliquidmeasurements, theQCMwas thermally equilibratedto 20 °C, prior to injection of liquid (3ml delivered via syringe) to entire-ly cover the crystal surface. Equilibrium was proceeded by an initialfrequency drop due to liquid loading, once stable the temperature wasincrementally raised to 40 °C and 60 °C.

2.6. AFM imaging and UFM nanomechanical mapping in ambient andunderliquid environments

UFM, which has been extensively discussed elsewhere [33], relieson ultrasonic vibration of the sample and subsequent nonlineardetection of the HF-modulated instantaneous forces acting on thetip as a consequence of the applied modulated vibration. The trans-mission of such vibration is performed via a UFM stage, which iscomposed of a piezoceramic disc [34] (4 MHz thickness mode reso-nance, PI Piezomaterials) covered with a cyanoacrylate bondedglass coverslip. Samples were attached to the glass coverslip using

Table 1Determination of graphene average thickness using QCM measurements.

GO rGO CDG

Average thickness (nm) 1422 ± 0.61 704 ± 0.45 15.6 ± 0.31Layer count 2031 ± 3 2070 ± 5 46 ± 1

phenyl salicylate (salol) by heating them up to 45 °C prior coolingto room temperature to induce recrystallization [20]. The AFM systememployed was a Multimode Nanoscope III (Bruker) and AFM probeswere common uncoated silicon probes (Contact-G, Budget Sensors).The amplitude of UFM response monotonously increases with thesurface stiffness, allowing the surface mapping of nanomechanicalproperties for a wide range of materials and has been applied in bothpolar (water) and non-polar (dodecane) liquid environments [23,35]allowing the investigation of tip–liquid–graphene and graphene–goldinterfaces in liquid environments.

3. Results and discussion

3.1. Scanning probe microscopy nanomorphological investigation ofgraphene films

Fig. 2 shows representative contact AFM morphology and UFMnanomechanical mapping of GO and rGO and CDG; GO featuresgranular-like surface morphology structure, most likely due tographene flakes agglomerations, whilst rGO clearly shows a higherdegree of order, where crystallites with similar orientation can beeasily observed. The GO agglomeration is driven by the tendency ofthe oxygen functionalities to form local oxidised domains resulting

Table 2Comparison of different average thickness measurement techniques for CDG films.

QCM Optical density [nm] AFM[nm]

Raman[nm]

Average thickness, nm 15.6 ± 0.31 4.3 ± 0.5 15 ± 5 ∼0.34–0.68Layer count 46 ± 1 13 ± 1.47 44 ± 14.7 1–2

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in disordered restacking of the flakes [36]. Conversely, the CDG filmis flatter which is typical of segregation growth [16,33], howeverwe note a number of brighter, i.e. protruding, features as highlightedby the arrows in Fig. 2e). Simultaneous contact stiffness mapping viaUFM (Fig. 2f), where darker contrast correspond to less mechanicallystiff regions, indicates that these higher features aremost likely areasof delaminated graphene arising from bulging of the film during

Fig. 5. QCM frequency shift time evolution for a CDG coated crystal exposed to (a) waterand (b) dodecane vapour.

transfer, i.e. the graphene film that is locally mechanically decoupledfrom the underlying gold substrate [22,33,37]. The delaminated re-gions were analysed as a percentage of total area for multiple scansacross the sample and found to comprise between 8 to 40% of thetotal film area.

In addition, folded regions were also observed with an averageheight, with respect to the local supported graphene film surface, of13.7 ± 1.8 nm; this is in good agreement with the values obtained viaQCM, below. CDG surface roughness was highly variable, with RMSvalues of ~2 nm far from the film edge, 3.5 nm near the edge and ca.7 nm on the folded regions, when measured over surface area of1 × 1 μm2, however it should be noted that this was substantiallylower than values for GO and rGO, which were found to be ca. 54 nmand 12 nm respectively.

The same samples were subsequently scanned in water Fig. 3 anddodecane environments. Whilst the topography did not change appre-ciably for GO, rGO exhibits increased inhomogeneity relative to ambientconditions (Fig. 3a); also CDG, despite comparable topography, demon-strated more homogeneous UFM signal in liquid than air, suggestingmore uniform contact with the substrate; both these are consistentwith liquid penetration between thin graphene films and substratereported previously[23].

3.2. Determination of graphene thickness; comparative analysis of QCM,Optical transmission, Raman, and AFM profilometry methods

Whilst micro-Raman spectroscopy [38], optical transmission [39,40]and AFM topography [41] are routinely used to determine graphenefilm thicknesses, these methods cannot be directly applied to thickor multilayer graphene films such as those explored in this study. Wetherefore used the QCM approach, widely employed in monitoringfilm growth during vacuum deposition, as a benchmark method fordetermining transferred graphene film thicknesses. Assuming thatgraphene acts as a rigid filmwhen attached to the quartz crystal surface,decrease of the resonant frequency relative to the unloaded crystal isdue solely tomass loading [29], and is described by the Sauerbrey equa-tion [42] to quantify themass of graphenefilm, and, correspondingly, itsthickness d:

d ¼ ΔmρGA

¼ Δ fffiffiffiffiffiffiffiffiffiffiρqηq

p2 f 0

2ρGð1Þ

whereΔf is the frequency shift in Hz,Δm is the corresponding change inmass per unit area in g cm−2, ρG is the density of graphite in g cm−3, A isthe active area in cm2, defined as the section of the crystal area wheretop and bottom electrodes overlap, f0 is the fundamental resonant fre-quency of the crystal, ρq is the density of quartz and ηq is the shearmod-ulus of quartz. Average thicknesses for GO, rGO and CDG films obtainedby QCM are shown in Table 1.

As indicated by QCM (Table 1), GO and rGO demonstrate film thick-nesses equivalent to over 2000 graphene monolayers which musttherefore be considered as a bulkmaterial; however due to the apparentagglomeration of the GO film prior to reduction it is likely that the thickrGO film is structurally distinct from bulk graphite of comparable thick-ness. The thinness film obtained, CDG, was subsequently used forcomparative thickness studies. Optical thickness determination wasperformed on the area of the film where graphene was available overthe optically transparent quartz surface. The absorption of a beam of

Table 3Frequency shifts, mass of absorbed vapour and characteristic tome for adsorption and desorption upon exposure of GO, rGO and CDG films to DI water vapour at 60 °C.

Frequency shifts(water evaporation)

GO rGO CDG Au coated crystal

Δfwv [Hz] 55.1 ± 1.8 37.7 ± 1.7 16.2 ± 1.5 36.0 ± 2.1Mass [ng] 332.4 ± 11 228.9 ± 10.2 97.7 ± 9.2 217.7 ± 13.3n. water layers 36.1 ± 1.2 24.9 ± 1.1 10.6 ± 1 23.7 ± 1.5n. GR layers 2031 ± 3 2070 ± 5 46 ± 1 –Water layers/GR layers 0.018 0.012 0.231 –τwv [s] 19 ± 2 26.5 ± 4.5 21.5 ± 0.5 22.4 ± 5.7Δfwd [Hz] 42.26 ± 1.85 32.42 ± 1.6 22.55 ± 1.52 26.90 ± 2.20Mass [ng] 255.1 ± 11.2 196.6 ± 9.7 136.3 ± 9.2 162.6 ± 13.3n. water layers 29.7 ± 1.2 22.9 ± 1.1 15.8 ± 1 18.9 ± 1.4Water layers/GR layers 1.462 × 10−2 1.106 × 10−2 3.438 × 10−1 –τwd [s] 12.5 ± 2.5 10.5 ± 0.5 13.3 ± 3 10.2 ± 1.7

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light in a medium is described by the Beer–Lambert law, which can bewritten as follows [43]:

I dð Þ ¼ I0 1−Rð Þe−αnd ð2Þ

where I(d) is the transmitted light intensity as a function of the thick-ness d, I0 is the incident light intensity, n is the refractive index, R isthe fraction of reflected light and α is the absorption coefficient, definedas:

α ¼ 4πkλ

ð3Þ

with k extinction coefficient and λ the wavelength of the laser source.For graphene it is usually assumed, in the visible range [39] (λ =670 nm), that reflection is negligible; thickness can be calculated withthe following simplified equation:

d ¼ − 1nα

lnII0

� �ð4Þ

The refractive index n and the extinction coefficient k values chosenfor the calculation were 2.6 and 1.3 respectively [40].

Raman spectra investigation, performed similarly to ones reportedelsewhere [16,33]; estimated the thickness by calculating the ratio I2D/IG for the 2D and G peaks. AFM profilometry was performed overmulti-ple line scans across the gold/graphene interface; a representativeimage is shown in Fig. 4.

Table 2 shows average thicknesses determined by the three differentmethods for the same CDG film. For CDG we observe good agreementbetweenQCMandAFMdata (∼15 nm),whereas opticalmeasurements,determined without the reflected component, returned a value about athird of this. Such a mismatch for the AFM measurements might origi-nate for several reasons, mainly from PMMA residue left from thegraphene transfer process and by the folded geometry of the film thatcan be observed in the AFM and UFM images (Fig. 1e,f). Such folding

Table 4Frequency shifts, mass of absorbed vapour and characteristic tome for adsorption and de-sorption upon exposure to dodecane vapour at 60 °C.

Frequency shifts(dodecaneevaporation)

GO rGO CDG Au coatedcrystal

Δfdd [Hz] 1.3 ± 2 2.7 ± 2.1 3.7 ± 1.7 0.8 ± 2.2Mass [ng] 7.8 ± 11.9 16.3 ± 12.6 22.2 ± 10 4.7 ± 13n. dodecane layers 1 ± 1.4 2 ± 1.5 3 ± 1.2 1 ± 1.5n. GR layers 2031 ± 3 2070 ± 5 46 ± 1 –Dodecanelayers/GR layers

4.923 × 10−4 9.658 × 10−4 6.527 × 10−2 –

τdv [s] 20 ± 1 21.7 ± 0.3 20.5 ± 2.5 37.8 ± 11.2

also indicates some degree of heterogeneity in the film thickness arisingfrom the film transfer processes. If we assume 15.6 nm to be the truethickness and use the full formulation of the Beer–Lambert law(Eq. (2)), that 18% of the incident light is reflected for the CDG structure.

Surprisingly, also for CDG, Raman thickness measurements weresubstantially different from the QCM and AFM values. Whilst manyreports in the literature rely on thickness determined by Raman spectra[44–46], such a techniquemay fall short in the characterisation ofmass-produced graphenewhere turbostratic structures can return ambiguousoutcomes [47].Moreover, the thickness of an underlying substrate playsa key role in the identification of the number of layers of graphene, as itis the case for graphene transferred on SiO2/Si substrate, where the I2D/IG ratio increases with SiO2 thickness [48]. Additionally, it has beenshown [38] that for more than 5 layers, Raman signature does notseem to provide valuable information for graphene layer count; more-over, the Raman spectrum of CDG graphene shows only a small Dpeak at ∼1370 cm−1 indicative of a certain degree of disorder in thestructure [16].

In conclusion, we believe that QCM and AFM combined could pro-vide rapid and accurate probing of large-area graphene film thickness,especially for thicker films, as well as morphology in industrially scal-able production routes such as those considered for this study.

3.3. Interaction of graphene with vapours of polar and non-polar liquids

QCM resonance frequency was used to monitor the adsorption anddesorption of polar (water) and non-polar (dodecane) vapours to thegraphene and graphene oxide films. To ensure rapid thermal equilibri-um, the QCM chamber was made of aluminium (see details in theMethods section), within the chamber a filter paper discwasmoistenedwith either DI water or dodecane whilst monitoring the frequency re-sponse. By comparing the frequency response curves for GO, rGO,CDG, and bare Au QCM, we found that the time evolution of such a re-sponse depended mostly on the liquid employed rather than the typeof coating. Typical QCM frequency responses are shown in Fig. 5.

On exposure to water vapour, for all surfaces studied, a rapid fre-quency decrease was observed, followed by exponential-like recovery(trend 1 in Fig. 5a) with a time constant τwv on the order of 21 s(Table 3). After a period of stabilisation (approx. 500 s), the frequencyrecovered to the pre-exposure value (trend 2 in Fig. 5a) over time τwdcorresponding to complete desorption of the water vapour. The resultsof exposure to dodecane vapour proceeded in a quite different way,with recovery occurring in single step with time constant τdv (Fig. 5b).Calculated time constants for GO, rGO, CDG, with water and dodecaneare presented in Tables 3 and 4 respectively. The initial frequencydrop, on exposure to vapour, ismost likely linkedwith the condensationof excess liquid on the surface of the graphene layers whilst, simulta-neously, the water was partially adsorbed in the graphene layer (as il-lustrated in schematically in Fig. 6). Subsequently, the evaporationand desorption of the liquid contained between the graphene flakes

Fig. 6. Two-step frequency trend for CDG uponwater exposure. (1) Frequency drops as a consequence of the condensation of water vapour into liquid on the surface; (2) frequency risesdue to equilibration of liquid layer previously condensed ongraphene surfacewith the gas phase; (3) diffusion of liquid throughgraphene layers; and (4) restoration of initial pre-exposureconditions due to evaporation of both adsorbed and diffused liquid.

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for GO and rGO, and graphene layers for CDG (step 4, Fig. 6) restore theQCM to the initial frequency within the error associated with the tem-poral stability of the crystal's frequency.

Liquid intake by the graphene and graphene oxide layers wasquantified as the frequency differences, Δfwv and Δfwd, correspond-ing to the liquid adsorbed upon exposure and the amount releasedthroughout evaporation (Table 3). Water absorption per layer ofgraphene was higher for GO (0.018 water monolayers/graphene

Fig. 7. Experimental measurement of (a) time evolution of frequency response to bubble nucle(b) time evolution of frequency response indodecaneat 60 °C for anAu crystal (black line), chemin % for CDG graphene (red dots) and an Au quartz crystal (black squares) in water at 60 °C.

monolayer) compared to rGO (0.012), whilst, surprisingly, high forCDG (0.231). The reference absorption of Au coated crystal was 24monolayers of water.

Table 4 summarises the results for non-polar dodecane vapour ex-posure the recorded frequency shift is within the measurement uncer-tainty (±5 Hz); therefore all graphene and graphene oxide filmsexperienced a much smaller total intake of dodecane vapour of below3 monolayers.

ation for CDG graphene (red line) and an Au quartz crystal (black line) in water at 60 °C,ically reduced graphene (red line) and grapheneoxide (blue line) and (c) bubble coverage

Fig. 8. Schematic of bubble nucleation on graphene-coated quartz crystals, interpreted from the experimental data shown in Fig. 7. (1) Bubbles begin to form at the solid/liquid interface;(2) bubbles grow bigger and form a cluster that steadily grows in size; (3) once the cluster has reached a critical size, bubbles begin to detach from the cluster at a similar rate to that atwhich they group together, given the symmetry of the curve; and (4) frequency returns to initial value as soon as all the bubbles have left the solid/liquid interface.

38 R. Mazzocco et al. / Thin Solid Films 585 (2015) 31–39

3.4. Graphene interaction with polar and non-polar liquids

Interfacial phenomenaoccurring betweengraphene and a surround-ing polar (DI water) and non-polar (dodecane) liquid environmentwere also investigated and indicated intriguing behaviour. Frequencyresponse was measured in liquids at 20 °C, 40 °C and 60 °C and refer-enced against Au coated QCM crystal at the same temperatures. At60 °C in water environment, we note periodic frequency peak oscilla-tions with a very long period of approximately 4000 s with amplitudedecaying over time and baseline frequency shift against dry QCM crystalΔf of −669 ± 3.5 Hz (Fig. 7a).

Additional measurements conducted at 40 °C and 80 °C showed thatthe period of these peaks increased to ∼14,000 s at 40 °C and decreasedto ~2100 s at 80 °C. We attribute the frequency oscillations to periodicbubble nucleation at the crystal/liquid interface [27,49,50] that reducesthe effective mass load on QCM. Independent confirmation was obtain-ed by manipulation of the chamber pressure resulting in a modulationthe bubble volume; increased pressure saw a corresponding immediatedecrease in bubble volume (as a function of surface coverage) whilstdecreased pressure resulted in larger bubbles some of which weredetaching from the surface and which were visible to the naked eye.The bubble formation cycle, corresponding to the experimental data inFig. 7, is shown schematically in Fig. 8.

Amplitude decay for the frequency peaks induced by the formationof bubbleswas plotted as a percentage of frequency upshift with respectto the crystal resonant frequency prior liquid injection and is shown inFig. 7c; at the maximum bubble coverage, ~20% of the CDG film surfaceis no longer in direct contactwith the surrounding polar liquid. It shouldbe noted that the frequency maxima (corresponding to the maximumamount of bubbles (point 2, Fig. 8)) decayed over time, consistentwith water partial degasification throughout the experiment. However,a significant feature of the dependence in Fig. 7a is that the lowestboundary of frequency shift Δf (marked by dashed lines and corre-sponding to point 4 of Fig. 8) was constant over all oscillations. Thisshift (and point 4 in Fig. 8) can be interpreted as the absence of the bub-bles, and therefore serve as a valuable parameters of the liquid loadedQCM crystal. Much smaller frequency oscillation, believed to have a dif-ferent origin of small temperature oscillations of the hot-plate duringdata acquisition, was observed in non-polar environment (Fig. 7b),this suggests that the bubble formation minimised the hydrophobicgraphene–polar liquid interface.

Here, water loading of the CDG graphene covered crystal produced afrequency shift of 524± 4 Hz that was ~40% lower than the 862± 3 Hz

Table 5Frequency shifts arising fromwater-film interactions referenced against Au coated crystalinwater (difference between graphene/graphene oxide coated crystal and Au coated crys-tal water loading).

Frequency shifts(water loading)

GO(r.a.Au)

rGO(r.a.Au)

CDG(r.a.Au)

Au coated crystal

Δfwload [Hz] 1611 ± 6 1429 ± 8 −338 ± 5 862 ± 3

frequency shift of the Au coated QCM in water indicating that either theshear wave decays in the graphene layers or that there is a certain tan-gential velocity slip of the liquid–graphene interface. The number ofwater layers was calculated to be 248 ± 4 and the water-layer-to-CDG-layer ratio was 5.4. For GO and rGO, the higher frequency shift(see Table 5) suggests that some liquid may indeed have diffusedthrough the graphene layers, thus increasing the mass of the graphenestructure rather than just introducing a liquid load atop the film. Thenumbers of water layers for GO and rGO were 1182 ± 6 and 1048 ± 7respectively, whilst the water-layer-to-graphene-layer ratios were0.58 and 0.51.

Dodecane loading of CDG graphene covered crystal lead to a totalfrequency shift of 600.8 ± 6.2 Hz, comparable with that of Au coatedQCM in dodecane of 600.5 ± 4.8 Hz suggesting that dodecane did notdiffuse through the graphene layers. However, the number of dodecanelayers that penetrated the CDG structure is significantly less than theerror associated with the measurement with a dodecane layer-to-CDG-layer ratio as low as 0.01, implying that no penetration tookplace. TheKanazawa equation [51] describes theQCM frequency changedue solely to liquid loading. Here, the Kanazawa equation returned avalue of 547 Hz for Au coated QCM in dodecane, within 9% of the actualmeasurement. As for GO and rGO films, dodecane loading values wereobserved to be higher than that of Au coated QCM, with additional826± 4 Hz and 845± 5 Hz frequency shifts respectively. These figuressuggest that an additional mechanism to liquid loading was present(Table 6). The dodecane penetration into the film is equivalent to287 ± 8 and 312 ± 9 dodecane layers in rGO and Go respectively, or0.14 and 0.15 dodecane layers-to-GR/GO-layer respectively.

4. Conclusions

We have combined QCM, AFM and UFM, to investigate physicalchanges in graphene-like samples grown via possible industriallyscalable routes such as carbon segregation growth (CDG) and chemicalreduction of graphene oxide (rGO), in ambient (air), water anddodecane vapour, and polar (water) and non-polar (dodecane) liquidenvironments at ambient and elevated (60 °C) temperatures. We com-pared measurement of total thicknesses of the film via differentmethods and observed that QCM is the preferred approach for trans-ferred films that allows one to avoid artefacts associated with othermeasurement techniques and accounts for the possibility of contamina-tion during the transfer process. We found that exposure of graphenefilms to water vapours revealed diverse adsorption of 0.018, 0.012 and

Table 6Frequency shifts arising from dodecane–film interactions referenced against Au coatedcrystal in dodecane (difference between graphene/graphene oxide coated crystal and Aucoated crystal dodecane loading).

Frequency shifts(dodecane loading)

GO(r.a.Au)

rGO(r.a.Au)

CDG (r.a.Au) Au coated crystal

225.1 ± 6.3 244.9 ± 7.1 0.25 ± 7.9 600.5 ± 4.8

39R. Mazzocco et al. / Thin Solid Films 585 (2015) 31–39

0.23 of water monolayers per graphene layer and 4.9 × 10−4,9.6 × 10−4, and 6.5 × 10−2 for dodecane for GO, rGO, and CDG films re-spectively. We also found that absorption–evaporation of water fea-tures a two-step behaviour for graphene coated crystals suggestingpenetration of water in the graphene film; this was practically non-existent for non-polar dodecane vapour. In underliquid environmentswe identified the formation of bubbles at the CDG/water interfacewhere up to 20% of the surface area of the CDG filmwas covered by gas-eous nanobubbles; this is of considerable significance for proposedenergy storage applications where the precise graphene surface–elec-trolyte contact area is a crucial parameter. These findings were consis-tent with the nanoscale morphology studies of the films observed inAFM topography and UFM nanomechanical maps of the same films inthe relevant environment. GO and rGO showed increased inhomogene-ity relative to ambient conditions but CDGfilms demonstratedmore ho-mogenous nanomechanical profiles, both observations are explained bythe different propagation of liquids within the graphene films and be-tween the film and substrates.

Acknowledgements

Authors acknowledge the support from EU FP7 grants, GRENADA(246073) and FUNPROB (269169) and by the Engineering and PhysicalSciences Research Council via an Industrial CASE award for JDW (AwardNo 07001545). CB is supported by the Lloyd's Register Foundation. TheLloyd's Register Foundation helps to protect life and property bysupporting engineering-related education, public engagement and theapplication of research.

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Surface and interfacial interactions of multilayer graphitic structures with local environment1. Introduction2. Materials and methods2.1. Graphene and graphene oxide via modified Hummer method2.2. CDG graphene growth and transfer2.3. QCM setup: hardware and software description2.4. Vapour detection experiments2.5. Underliquid QCM measurements2.6. AFM imaging and UFM nanomechanical mapping in ambient and underliquid environments

3. Results and discussion3.1. Scanning probe microscopy nanomorphological investigation of graphene films3.2. Determination of graphene thickness; comparative analysis of QCM, Optical transmission, Raman, and AFM profilometry methods3.3. Interaction of graphene with vapours of polar and non-polar liquids3.4. Graphene interaction with polar and non-polar liquids

4. ConclusionsAcknowledgementsReferences