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Electrochimica Acta 132 (2014) 265–276

Contents lists available at ScienceDirect

Electrochimica Acta

j ourna l ho me page: www.elsev ier .com/ locate /e lec tac ta

nfluence of the laser irradiation on the electrochemical andpectroscopic peculiarities of graphene-chitosan composite film

aimonda Celiesiute a, Romualdas Trusovasa,b, Gediminas Niaurac,1, Vitas Svedasd,ediminas Raciukaitisb, Zivile Ruzele a, Rasa Pauliukaitea,∗,1

Department of Nanoengineering, Center for Physical Sciences and Technology, Savanoriu Ave. 231, LT-02300 Vilnius, LithuaniaDepartment of Laser Technology, Center for Physical Sciences and Technology, Savanoriu Ave. 231, LT-02300 Vilnius, LithuaniaDepartment of Organic Chemistry, Center for Physical Sciences and Technology, A. Gostauto 9, LT-01108 Vilnius, LithuaniaLaboratory of Applied IR Spectroscopy, Center for Physical Sciences and Technology, Savanoriu Ave. 231, LT-02300 Vilnius, Lithuania

r t i c l e i n f o

rticle history:eceived 23 December 2013eceived in revised form 6 March 2014ccepted 19 March 2014vailable online 2 April 2014

a b s t r a c t

Activation of the chitosan-modified graphene cast on indium-tin-oxide electrodes was performed withlaser irradiation. The composite electrodes were characterised spectroscopically and electrochemicallyapplying Fourier transform infrared and Raman spectroscopies, cyclic voltammetry and electrochemicalimpedance spectroscopy, respectively. Significant rise in the double-layer capacitance was obtained afterthe laser irradiation of the modified graphene film formed from its dispersion in chitosan solution. Effect

eywords:raphenehitosanaser irradiation

of the laser irradiation depended on a load of the modified graphene in the film. The laser treatmentimproved electrochemical properties of the electrodes made of the modified chitosan-graphene-filmdue to creation of edge defects and formation of nanocrystalline structures from graphene flakes.

© 2014 Elsevier Ltd. All rights reserved.

lectrochemical impedance spectroscopyaman spectroscopy

. Introduction

Graphene is an attractive material since its discovery byovoselov et al. in 2004 [1]. Its unique structural, mechanicalnd electronic properties determine broad application possibilities2–6]. Use of graphene in electrochemical applications is a compli-ated task due to the requirements of a stable layer on an electrodeurface.

Lasers are yet not widely applied for graphene activation. Soar, this method was used for graphene synthesis [7,8], reductionf graphene oxide (GO) to graphene [9–11], reduction of graphenelm down to a monolayer [12], modification of graphene [13,14]nd laser patterning [15]. Graphene reduced from GO using a fem-osecond laser can also be applied to electroanalysis, for instance,

o the sensitive hydrogen peroxide detection [11].

Chitosan is a versatile biological compound obtained from chitinnd it is widely used in many fields due to its intrinsic properties

∗ Corresponding author. Tel.: +370 5 2644886; fax: +370 5 260 2317.E-mail addresses: celiesiute@ftmc.lt (R. Celiesiute), romualdas.trusovas@ar.fi.lt

R. Trusovas), gniaura@ktl.mii.lt (G. Niaura), svedas@ktl.mii.lt (V. Svedas),raciukaitis@ar.fi.lt (G. Raciukaitis), ruzele@ftmc.lt (Z. Ruzele), pauliukaite@ftmc.ltR. Pauliukaite).

1 ISE member

ttp://dx.doi.org/10.1016/j.electacta.2014.03.137013-4686/© 2014 Elsevier Ltd. All rights reserved.

allowing fabricate membranes, thin films, three-dimensional struc-tures as well as immobilise graphene and carbon nanotubes[16–18].

Influence of the laser treatment on carbon composites wasrecently reported in the literature. Carbon composites for opticalfibres presented much better mechanical properties after the lasertreatment than untreated ones [19]. Dittmar et al. applied nanosec-ond UV-laser ablation of carbon and glass fibre reinforced epoxyresin [20]. Results showed that nanosecond laser is suitable tool forcarbon reinforced plastics machining, however laser fluence andpulse overlap must be chosen carefully to achieve desirable pro-cess quality and reduce thermal effects. The pulse laser ablation ofbulk graphite in tetraethyloxysilane under rather long pulse dura-tion and low power density caused the simultaneous formation ofpolyynes and carbon nanoplates/ribbons composed of graphene-based lamellae with Si–H dopant [21]. Effect of the laser interactionon carbon composite most depends on laser parameters such asenergy, power and pulse duration [22].

In the present work, a strategy of the graphene film activationwith a picosecond laser is described. The homogeneous dispersionof the functionalised-graphene in aqueous chitosan solution was

chosen as a matrix as its advantages were shown in the previouswork [23]. Graphene dispersion in chitosan solution was cast onITO surface by spin-coating technique and treated with a picosec-ond laser in order to reduce the film thickness and increase its

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66 R. Celiesiute et al. / Electroc

lectrochemical activity. The efficiency of the laser activation wasvaluated spectroscopically and electrochemically.

. Experimental

.1. Materials and Chemicals

Graphene flakes with thickness of 8 nm were obtained fromraphene Supermarket (USA). Indium-tin-oxide (ITO) glass slides00 × 100 mm2, resistivity 12 � cm−2 were purchased from Opticalilters (UK). The slides were laser cut into square electrodes withhe size of 20 x 20 mm2.

K4Fe(CN)6·3H2O, CH3COOH, KCl, and chitosan from shrimphells were obtained from Sigma Aldrich (Germany). H2SO4, HNO3,aH2PO4·H2O, Na2HPO4, and NaOH were obtained from ROTHmbH (Germany). All reagents used were of analytical grade. Allolutions were prepared using ultrapure MilliQ-water (resistivityf 18.2 M� cm) directly taken from the Synergy 185 unit equippedith a UV lamp (Millipore, USA).

.2. Methods and apparatus

The laser activation of the films was performed with the picosec-nd laser Atlantic (Ekspla, Lithuania) with the pulse duration of0 ps and pulse repetition rate of 100 kHz. The wavelength ofhe radiation was 1064 nm. Focused laser beam was scanned overhe sample surface. Beam spot diameter at the focus was 20 �m.he scanning speed was varied from 50 to 300 mm s−1, and thepplied mean laser power was varied from 50 mW to 200 mW,xceptional cases are specified in the text. These two parametersontrolled the laser irradiation dose of the film. The accumulatedaser irradiation dose in J cm−2 was estimated from a laser flu-nce, which was proportional to the mean laser power at the fixedulse repetition rate and spot diameter, multiplied by the num-er of laser pulses (depending on the scanning speed) affecting theample.

Raman measurements were performed with the Raman micro-cope inVia (Renishaw, UK) equipped with the thermoelectricallyooled CCD detector using four different excitation wavelengths:42 nm (0.8 mW) line from a He-Cd laser, 532 nm (0.6 mW) from aiode-pumped solid state laser, 633 nm (0.5 mW) from a He-Ne

aser, and 785 nm (1.8 mW) from a diode laser. Raman spec-ra were taken using a 50x/0.75 NA objective lens. Integrationime was 50 s. Wavenumber axis was calibrated according to thei line at 520.7 cm−1. Raman measurements were carried out in80◦ (backscattering) geometry. Frequencies and intensities of theaman bands were determined by fitting the experimental contourith the Gaussian-Lorentzian form components. Spectral analysisas performed by using GRAMS/A1 8.0 (Thermo Scientific, USA)

oftware.FTIR spectra were measured with the Nicolet 8700 spectrometer

Thermo Scientific, USA) in the range of 500-7400 cm−1 in transmis-ion mode with subtraction of the polyethylene film absorption.

The scanning electron microscope JSM-6490 LV (JEOL, Japan)as used for investigation of surface morphology before and after

he laser treatment. Samples were investigated by SEM withoutny additional pre-treatment. The thickness measurements wereerformed with a Dektak 150 + stylus profiler (Veeco, USA).

Cyclic voltammetry (CV) and electrochemical impedancepectroscopy (EIS) measurements were conducted with theompactStat potentiostate/galvanostate with impedance module

Ivium Technologies, The Netherlands). The three-electrode sys-em was used employing the bare or graphene modified ITO as aorking electrode, Pt wire was as a counter electrode and Ag/AgCl

KCl sat.) served as a reference. EIS was performed with the same

Acta 132 (2014) 265–276

equipment at a constant applied potential in a frequency range from100 kHz to 0.1 Hz, with potential perturbation of 10 mV.

CVs at the bare and modified ITO were recorded in the potentialrange from -1.0 to 1.0 V with the start potential at 0 V in 0.1 M KCl.Potential sweep rate was 0.1 V s−1. EIS spectra were recorded atthe bare and modified ITO before and after their irradiation withthe picosecond-laser at various potentials in the whole potentialrange used in CV investigation: -0.75; -0.50; 0.00; 0.50; 0.75; 1.00 V.The lowest potential -1.00 V was not applied because the spectrawere scattered due to the hydrogen evolution at this potential. Thegraphene load was varied from 0 to 3 mg mL−1.

2.3. Preparation of graphene-chitosan film

Graphene flakes were additionally exfoliated to thickness of2-3 nm and functionalised with hydroxy- and carboxy-groups bysonication 50 mg of graphene flakes in the mixture of 5 mol L−1

H2SO4 and HNO3 3:1 (V:V) at 40 ◦C for 20 h. The mixture was fil-trated and washed excessively with the MiliQ water. Finally, a solidprecipitate was dried at 80 ◦C for 24 h.

Aqueous solution of 0.5% chitosan was prepared as describedelsewhere [24]. Chitosan was dissolved in aqueous 1% CH3COOHsolution; then pH was adjusted to 5.0 with 20% NaOH solution.Then functionalised graphene was added to the solution and soni-cated for 2 h to reach a homogeneous dispersion. Suspensions ofappropriate concentrations (10 pg mL−1, 10 �g mL−1, 1 mg mL−1,and 3 mg mL−1: G1, G2, G3, and G4, respectively) were prepared inthis way. One of these suspensions was spin-coated on the indium-tin-oxide (ITO) square electrode with the WS-650-23 spin-coater(Laurell Technologies Corporation, USA), at the 700 rpm rotationspeed for the period of 1 min and then 1800 rpm for 30 s. The elec-trodes were left to dry overnight and then the characterisation wasconducted applying CV and EIS. Thickness of the graphene-chitosanfilm was ∼1 �m.

The control sample for the FTIR measurement was prepared asfollows: solution of chitosan and suspension of G2 in chitosan solu-tion (as described above) were covered on the framed 16 �m-thickpolyethylene film and dried in air. The thickness of the depositedfilm was measured with the bench micrometer and the thickness of10 and 6 �m was found for chitosan and G2-Chit films, respectively.

3. Results and discussion

3.1. Optical investigation of graphene-chitosan film modified ITOelectrodes

3.1.1. Raman spectroscopyRaman spectroscopy is a non-destructive and highly sensitive

technique for characterisation of the structure and defectivenessof carbon materials [25,26]. The characteristic D, G and 2D bandsare visible in Raman spectra around 1333, 1580, and 2680 cm−1,respectively, using excitation with the wavelength of 633 nm. TheG band of E2g symmetry was assigned to in-plane relative motionof pairs of carbon atoms in sp2 hybridization [25]. The D mode hasA1g symmetry and arises from the breathing vibration of aromaticrings. Defects are required for activation of this mode, while, nodefects are necessary for the activation of the second order of Dpeak, the 2D mode. Position of both D and 2D modes depends onthe excitation wavelength [25–28]. The ratio of the intensity of theD and G bands, I(D)/I(G) is useful for characterisation of the defectcontent in the sample. Development of two kinds of defects might

be responsible for increase in I(D)/I(G) ratio; i.e. the point defectsassociated with changes in sp2 hybridisation in the carbon latticeplane, and formation of new edges due to the reduction of averagesize of the graphene flake [29,30].

R. Celiesiute et al. / Electrochimica Acta 132 (2014) 265–276 267

Fig. 1. Raman spectra of (a) modified and (b) pristine graphene flakes. The differencespectrum is also shown (c). Excitation wavelength is 632.8 nm (0.5 mW). Spectra arenv

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tsptspultrbpa1sia6

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Fig. 2. Raman spectra of the G3-Chit/ITO sample: (A) irradiated with the mean laserpower of 200 mW; (B) before the laser treatment; and (C) difference spectrum.Excitation wavelength was 632.8 nm (0.5 mW). In construction of the difference

ormalised according to the intensity of the G band at 1578 cm−1. Spectra are shiftedertically for clarity.

Fig. 1 compares Raman spectra of pristine and functionalisedraphene flakes obtained with the 633 nm excitation wavelength.he prominent D, G, and 2D bands appeared at 1333, 1578,nd 2660/2682 cm−1, respectively. The low intensity shoulder at616 cm−1 was due to the defect-induced mode D′. The I(D)/I(G)atio for the pristine sample was found to be 0.17, indicating rela-ively low number of defects in the sample. The difference spectrumFig. 1(c)) clearly showed an increase in intensity of the D bandpon functionalisation of the graphene flakes, and the I(D)/I(G) ratio

ncreased to 0.23. Thus, the functionalisation procedure resulted in slight increase in defect concentration. The nature of the defectsan be evaluated by the analysis of the relative intensity I(D)/I(D′),hich was found to be 3.8; characteristic value for dominating

oundary-like defects [31]. In addition, the difference spectrumndicated the decrease of the 2D band component correspondingo the single-layered graphene at 2660 cm−1.

The laser treatment resulted in a considerable broadening ofhe first order Raman bands (Fig. 2). Comparison of the Ramanpectra before and after the laser treatment revealed that only theart of the initial graphene flakes are subject to the transformationo more disordered carbon structures. Fitting of the experimentalpectrum with Lorentzian-Gaussian form components showed theresence of the sharp bands located at the same wavenumbers asntreated sample (1335 and 1581 cm−1). Thus, the spectrum of the

aser activated carbonaceous material can be obtained subtractinghe spectrum before the laser irradiation (Fig. 2A). Vallerot et al.ecently demonstrated that the accurate analysis of the pyrocar-ons the first order Raman spectra are obtained when the fitting iserformed with the five components, representing the I, D, D′ ′, G,nd D′ bands located at 632.8 nm excited spectra near 1170, 1330,500, 1580, and 1618 cm−1, respectively [32]. Thus, the differencepectra in this work were fitted with the five components, assum-ng the constant frequencies for I (1170 cm−1), D′ ′(1500 cm−1),nd D′ (1619 cm−1) bands for the data obtained with excitation of

32.8 nm.

The laser treatment effect on the Raman spectra of the G4-hit/ITO and G3-Chit/ITO samples is demonstrated in Figs. 3A and B,

spectra the intensities were normalised according to the G-band near 1581 cm−1.The background of the spectra was corrected by a polynomial function.

respectively. Intensity of the G band decreased after the laser treat-ment because of conversion of the initial relatively low-defectedgraphene structures to disordered forms. For example, laser treat-ment with the mean power of 200 mW (accumulated irradiationdose of 4.46 J cm−2) resulted in the decrease of the G peak intensityby a factor of 2.0 for the sample G3-Chit/ITO (Fig. 3B). Thus, roughlyhalf of the initial graphene flakes were converted to more disor-dered structure material. The difference spectra were extracted inorder to access changes in the microstructure of carbon compositesinduced by the laser treatment. These spectra revealed develop-ment of broad D and G features when the large laser irradiationdoses were applied. The width of the D and G bands determinedas full width at half maximum (FWHM) was found to be 206-261and 95-199 cm−1, respectively. FWHM of the G mode (FWHMG) is ameasure of the bond-angle disorder in sp2 hybridised carbon struc-tures [25]. For example, FWHMG of around 180 cm−1 was detectedfor sputtered amorphous carbon film by using the 633 nm laserexcitation line [26]. The FWHM of D band (FWHMD) was foundto be sensitive the in-plane structural order, i.e. disorientations ofthe graphene layers, atomic dislocations, and other in-plane defects[32]. In this work, the high FWHMD values observed were similar tothose for regenerative laminar polycarbon structure (170-200 cm-1), which characteristics are high anisotropy and a large number ofdefects [32]. Thus, appearance of such broad bands was evidencethat the laser treatment induced formation of highly disorderedcarbon clusters. In the case of the sample G4-Chit/ITO (Fig. 3A) pre-pared with the relatively low laser power of 50 mW (irradiationdose of 1.11 J cm−2), the difference spectrum showed development

of one broad feature (FWHM = 199 cm−1) centred at ∼1484 cm−1.Such spectral shape can be explained by shift of the G peak towardslower wavenumbers. The detailed Raman spectroscopic analysis of

268 R. Celiesiute et al. / Electrochimica Acta 132 (2014) 265–276

Fig. 3. Raman spectra of the (A) G4-Chit/ITO and (B) of the G3-Chit/ITO samples irradiated with different mean laser power: (a) 0, (b) 50, (c) 100, and (d) 200 mW. Differences ent.

t ion an

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pectra are also shown. The scanning speed was 300 mm s−1 during the laser treatmhe G-band near 1581 cm−1. Spectra are background corrected by polynomial funct

morphous and disordered carbon by using the three stage modelave revealed softening of the G peak upon formation of amor-hous carbon from nanocrystalline graphite [25,26]. Similarly, aingle broad Raman peak was observed recently upon amorphi-ation of the functionalised graphene sample by treatment with aocused ion beam [33]. Considerably lower wavenumber of the Geak (1552 cm−1) comparing with the initial sample (1581 cm−1)as also visible in the case of the treatment with the 100 mW laserower (irradiation dose (2.23 J cm−2) as seen in Fig. 3A. Using theigher mean laser power, presence of the D and G peaks was clearlyisible and the I(D)/I(G) ratio was found to be close to 2 indicatingormation of the highly disordered nanocrytalline graphite.

The multiwavelength Raman spectroscopy provides a possibil-ty to distinguish the degree of disorder in carbonaceous materialsy analysis of dispersion of the G peak [25]. Only in the highly dis-rdered carbon, the G peak shows dispersion which is proportionalo the degree of disorder, while, no such dispersion was observedor crystalline graphite, nanocrystalline graphite or glassy carbon25,34]. Fig. 4 compares the difference Raman spectra measuredn the 900 − 2000 cm−1 spectral region using the four excitation

avelengths. Shift of the D band towards lower wavenumberss clearly visible with the increase in the excitation wavelength;o such shift was observed for the G band. Quantitative analy-

is of the peak positions revealed the dispersion of the D-peaks high as ∼53 cm−1/eV. This value is very similar to the disper-ion (51 cm−1/eV) observed for defected graphite obtained by itsrradiation with 2 MeV protons at a fluence of 1 · 1018 ions/cm2

In construction of difference spectra, the intensities were normalised according tod shifted vertically for clarity.

[35]. Absence of the G peak dispersion for the G3-Chit/ITO and G4-Chit/ITO samples indicated that the laser treatment with the 200mW mean power (irradiation dose (4.46 J cm−2) resulted in for-mation of disordered nanocrystalline graphite, but not amorphouscarbon. As known, the relative intensities of the Raman bands ofcarbonaceous material depend on the arrangement of graphenesheets and orientation of the polarisation direction of the irradi-ated laser beam [36–38]. The laser beam was linearly polarised inthe X-Y plane of the studied sample in these Raman investigations.The incident light was polarised parallel to the graphene planessince no sharp Raman line (A-band) near 867 cm−1 was observedin all samples studied in this work (Fig. 3). Kawashima and Kata-giri assigned this band to “out-of-plane” vibrational mode of highlyoriented pyrolitic graphite (HOPG). This band was observed onlyfor perpendicularly polarised laser light to the graphene planes inHOPG [39].

3.1.2. Fourier transform infrared spectroscopyThe samples of laser treated G-chitosan composite film prepared

for electrochemical investigation were not convenient as samplesfor a Fourier transform infrared spectroscopy (FTIR). The layer of thespin-coated G-Chit composite on the ITO/glass substrate influencedreflectivity of thin ITO layer. Spectral variation of the ITO reflectivity

caused by the composite layer on the top raised distortion of thebaseline of the FTIR measurement. Furthermore, the distortion ofthe baseline caused the shape distortion and shift of the peaks inthe spectra. In order to obviate the spectral distortions, a separate

R. Celiesiute et al. / Electrochimica Acta 132 (2014) 265–276 269

Fig. 4. Difference Raman spectra of the G3-Chit/ITO sample measured using excita-tion wavelength of 442 nm (2.81 eV), 532 nm (2.33 eV) 633 nm (1.96 eV), and 785 nm(1.58 eV) (upper) and dependence of the peak wavenumbers of the G and D bandsop

ss

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Fig. 5. FTIR spectra of (a) chitosan and (b) G3-Chit composite; both films weredeposited on polyethylene. In order to compare the peak height of the G3-Chit com-posite spectrum was multiplied by the sample thickness ratio 10: 6. Vertical offsetis added to the chitosan spectrum for clarity. The 2940-2840 cm−1 interval was cutout because of the strong polyethylene substrate absorption. Both composite peaksat 1018 cm−1 and at 993 cm−1 are resolved by the least square fit deconvolution ofthe 1200-970 cm−1 band by the six Lorentz shape peaks. New peaks emerged in the

flat with clearly visible influence of the graphene concentration in

n the excitation wavelength (bottom). The sample was treated with the mean laserower of 200 mW.

et of samples was prepared for FTIR analysis on the polyethyleneubstrate, as described in Experimental Section.

The FTIR spectra of chitosan and G2-Chit composite films areresented in Fig. 5. A few types of interaction between the func-ionalised graphene and chitosan were observed in the spectra.omparison of the peaks of chitosan and of the composite filmrovided an evidence of the hydrogen bonds rearrangement in chi-osan caused by addition of functionalised graphene, which leado the graphene side groups binding to chitosan by forming a netf graphene modified with chitosan. Similar results were obtainedith cellulose [40], which is chemically very close to chitosan.

he results of reference [40] allow assigning the peaks of chi-osan at 1043 and 1012 cm−1 to C-O stretch of -CH2-O-H primarylcohol group. Furthermore, these peaks corresponded to differentonformations of this group, conformation I and conformation III,espectively [40]. Addition of graphene caused shift of the peaks ofhe conformations I and III from 1043 cm−1 to 1034 cm−1 and from012 cm−1 to 1018 cm−1, respectively. Moreover, vibration of theonformation II of the primary alcohol group mentioned above at1000 cm−1 [40] was not observed in pure chitosan but it appearedt 993 cm−1 in the spectrum of the composite film.

After graphene addition, a new peak appeared at 1377 cm−1, andt can be attributed to the O-H bend vibration of tertiary alcoholroups [41] of the chitosan-modified graphene. The spectral peaks

spectrum of the G3-Chit composite are marked with the arrows. Chitosan primaryalcohol C-O vibration peaks which shift after graphene addition are marked withthe dashed lines.

of carboxyl in the functionalised graphene varied due to interac-tion with chitosan. As observed in [42], the peak at 1718 cm−1 ofCOOH group in the modified graphene in this work was shifted to1680 cm−1 in the G2-Chit composite film. In the same way, the peakat 1659 cm−1 in Fig. 5 arose as a result to downshift of the graphenecarboxyl vibration due to hydrogen binding with molecular chainsof chitosan.

Further, the peak at 1572/1579 cm−1 can be assigned to thechitosan amide II -NH2 group vibrations at ∼1590 cm−1 [43]. Thepeak observed at 1417/1413 cm−1 can be attributed to bendingvibrations of chitosan C-H and -CH2- groups as in [43]. More-over, C-H stretching vibration bands of chitosan [41] at ∼2990 and∼2770 cm−1 were present in both spectra (Fig. 5).

The FTIR spectra of G-Chit/ITO samples were also taken fromlaser treated and untreated areas; however, in all cases absorbancewas rather low as well as the baseline was complex due to the spec-tral variation of ITO reflectivity, as mentioned above. FTIR spectradid not show any remarkable chemical changes after the laser treat-ment (not shown). Nevertheless, the laser irradiation caused somephysical changes because significant decrease in the IR beam scat-tering was observed, which might be due to densification of thefilm. Scattering in the range of 7000-4000 cm−1 was several timeshigher in untreated area than in the laser irradiated ones. FTIR datashow that composite is still present after laser treatment as wasfound in [19].

3.1.3. Scanning electron microscopyIn order to evaluate surface morphology of the samples, SEM

images were taken before and after laser irradiation, and examplesare presented in Fig. 6. The laser untreated film was not perfectly

the film, (Fig. 6A,B). Roughness of the film surface increased withgraphene concentration. The laser irradiation caused some changesin the surface morphology of the films. A couple of examples are

270 R. Celiesiute et al. / Electrochimica Acta 132 (2014) 265–276

F ntrea1

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ig. 6. SEM images of (A,C,E) G3-Chit/ITO and (B,D,F) G4-Chit/ITO taken in (A,B) u.67 J cm−2 (C,D), and 3.34 J cm−2 (E,F), using 300 mm s−1 scanning speed.

resented in Fig. 5C-F, where the increase of roughness of the filmsaused by the laser irradiation was evident. The higher laser irra-iation dose led to a larger roughness of the film (Fig. 6 C,E or D,F).

Chitosan was likely ablated by the laser irradiation and grapheneakes were exposed with the increase in the picosecond laserower. Since more graphene edges were opened, the D-band wasnhanced in Raman spectra after the laser treatment. In addition,ccording to the Raman spectra, more edge defects were observedfter the laser treatment, which is in good agreement with theEM morphology images. On the other hand, the SEM imageshowed graphene flakes surrounded with polymer and this factlso confirmed the FTIR spectra of the composite film where clearonnection between graphene and chitosan was observed.

The data obtained are in a good agreement with the knownact that the laser interaction with carbon nanomaterial compositeeads to cleaning of materials, morphological changes and removal

f material at higher laser power density [22,44]. Efficiency of theaser processing strongly depends on the laser irradiation parame-ers such as fluence and pulse overlap (irradiation dose) as has beenound in [20].

ted areas and (C-F) irradiated with the picosecond laser with the irradiation dose

3.2. Electrochemical investigation of graphene-chitosan modifiedITO electrodes

3.2.1. Cyclic voltammetryFig. 7A presents CVs of bare and modified ITO recorded in

0.1 M KCl solution. The bare ITO electrode exhibited a reduc-tion wave at ca. -0.8 V in the scan going towards negativepotentials and this reduction was also observed at negativepotentials in the backwards scan but shifted towards less neg-ative potentials (Fig. 7A, curve (a)). The electrode modificationwith G2-Chit film caused a shift of the position of this reduc-tion wave to less negative potential values as seen from curve(b) in Fig. 7A, ca. -0.6 V, which was closer to reduction ofcarbon-oxy-species than that of ITO, indicating the presence ofgraphene. After the laser irradiation (Fig. 7A, curve (c)), the reduc-tion peak decreased significantly indicating the ITO coverage but

less carbon-oxy-species, maybe due to reduction of the modi-fied graphene as was found in other works [10,11]. Moreover, insuch a way an effective potential window of the electrode wasextended.

R. Celiesiute et al. / Electrochimica Acta 132 (2014) 265–276 271

Fig. 7. CVs at ITO (a), G2-Chit/ITO (b) and laser irradiated G2-Chit/ITO (c) in (A)0.1 mol L−1 KCl and (B) 0.1 mol L−1 KCl with 2 mmol L−1 K4Fe(CN)6. Potential scanrate was (A) 100 mV s−1 (B) 50 mV s−1. Inset (A): optical micrograph of G2-Chit/ITOuo

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Table 1Electrochemical performance of different G-Chit/ITO electrodes in 0.1 M KCl solutioncontaining 2 mM K4Fe(CN)6. Geometric area of the electrode exposed to the solutionwas 0.126 cm2. For all measurements 3 > n > 6. Laser scanning speed was 300 mms−1 during the irradiation.

Electrode Mean laserpower, mW

Film thickness,�m

AEA, cm2

ITO 0 - 0.084G1-Chit/ITO 0 0.935 ± 0.089 0.155

50 0.899 ± 0.052 0.089 ± 0.012500 0.731 ± 0.032 0.063 ± 0.008

G2-Chit/ITO 0 0.998 ± 0.091 0.156 ± 0.01850 0.876 ± 0.036 0.100 ± 0.015500 0.719 ± 0.044 0.065 ± 0.005

G3-Chit/ITO 0 1.00 ± 0.20 0.326 ± 0.02225 0.864 ± 0.086 0.311 ± 0.01850 0.832 ± 0.091 0.224 ± 0.01875 0.768 ± 0.038 0.198 ± 0.025100 0.734 ± 0.045 0.173 ± 0.016150 0.720 ± 0.082 0.067 ± 0.007200 0.637 ± 0.049 0.062 ± 0.017

G4-Chit/ITO 0 1.15 ± 0.14 0.357 ± 0.02625 0.817 ± 0.064 0.349 ± 0.04950 0.784 ± 0.080 0.285 ± 0.01075 0.701 ± 0.074 0.270 ± 0.005

5 1/2

ntreated area (right) and area treated with the picosecond laser using a mean powerf 50 mW (left).

Electrochemical behaviour of the well-known redox compoundotassium hexacyanoferrate(II) was also tested with the graphene-hitosan modified ITO electrode. The results are presented inig. 7B. The G-Chit coating on the ITO glass electrode led to anncrease in a peak current of the redox couples due to a higherlectroactive area but, on the other hand, the peak-to-peak sepa-ation increased significantly showing a diffusional barrier of thelectroactive species through the film. The peak-to-peak separa-ion decreased slightly from 120 to 80 mV with an increase in theraphene load in the film, however, it was higher than that at bareTO (70 mV), and higher than the theoretical value of 59 mV for ane-electron process. The peak-to-peak separation decreased afterhe laser irradiation and it depended on the mean laser powers well as the graphene load. This diffusional barrier was causedy a rather thick chitosan layer. After the laser irradiation, thelm was thinned by more than 200 nm, depending on the laser

rradiation dose, and the diffusion barrier decreased (Table 1). Nev-rtheless, with this decrease, the peak height also decreased, whichas almost the same like at bare ITO electrode (Fig. 7B, grey line)

howing some changes in the film structure at the interface and theecrease of electroactive sites caused by chitosan destruction.

The electroactive surface area for all electrodes was estimatednder the same conditions, i.e. in 0.1 mol L−1 KCl containing

mmol L−1 K4Fe(CN)6 by changing the potential scan rate. The

100 0.631 ± 0.004 0.220 ± 0.021150 0.588 ± 0.089 0.162 ± 0.011200 0.148 0.148

potential scan rate was changed from 0.005 to 0.3 V s−1. The cur-rent peak increased linearly with the square root of the scanrate over the whole studied range of the scan rate indicating thediffusion-controlled electrochemical process at all electrodes. Theelectroactive surface area was calculated using the slope of thedependence Ip vs v1/2 and the Randles-Sevcik equation [45]:

Ip = 268700 n3/2AEA c D1/2v1/2 (1)

where Ip is the peak current in [A], n is the number of electronsparticipating in electrochemical reaction, AEA is the electroactivearea of the electrode in [cm2], c is the bulk concentration of a redoxcompound in [mol cm−3], D is the diffusion coefficient of the elec-troactive species in [cm2 s−1], and v is the potential scan rate in[V s−1]. D was taken 7.6 · 10−6 for Fe(CN)6

4− in 0.1 M KCl [46]. Thedata of the electroactive area are given in Table 1. The geometricarea of the electrode exposed to the solution in electrochemicalmeasurements was equal to 0.126 cm−2.

As seen from Table 1, the electroactive area of the electrodesdepended on the graphene load: the higher the load, the higherthe AEA of the electrode. Moreover, it also depended on laser irra-diation conditions, i.e. the higher the used laser power, the lowerthe AEA. When the high laser irradiation dose (> 3.34 J cm−2) wasapplied to the film with lower graphene load, AEA was even smallerthan the geometric area exposed to the solution or at least it wasclose to the electroactive area of the bare ITO. Reversibility of theredox process also depended on the graphene load and laser irradi-ation conditions: most probably, this was related to the thicknessof the G-Chit film when electroactive species had to penetratethe chitosan net to reach graphene and, after the laser irradia-tion especially with the mean power > 100 mW (irradiation dose> 2.23 J cm−2), the reversibility improves significantly in terms ofthe decrease of the peak-to-peak separation down to 75 mV. Asexpected, the peak current ration also got closer to 1.0 showingimprovement in the reversibility of the electrochemical process.The charge transfer coefficient can be calculated for slow kineticsdiffusion controlled reactions applying following equations [46]:

Ip = 3 · 10 n (aDv) c (2)

where is the charge transfer coefficient and all other parame-ters are the same as in equation (1). ˛, calculated for ITO electrode,

2 himica

wcGtt

E

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Fg

72 R. Celiesiute et al. / Electroc

as equal to 0.51, which indicated the reversible fast electro-hemical reaction. After G-Chit deposition on ITO, was 0.77 for3-Chit/ITO and 0.60 for G4-Chit/ITO, but it decreased to 0.55 after

he laser irradiation due to the G-Chit film thinning. Subsequently,he rate constant, k0, was calculated according to [46]:

= −RT/anF [0.780 + ln(D1/2/k0) + 1⁄2 ln b] (3)

where b is taken from the determined parameters given in47]. k0 was 1.37 s−1 for the ITO electrode, showing a fast chargeransfer kinetics, but it increased significantly after the electrode

odification and was equal to 17.5 and 17.7 for G3-Chit/ITO and4-Chit/ITO, respectively. Such a slow kinetics is characteristic to

he thick-film behaviour when redox species have to penetrate thelm [46]. After the laser irradiation, the rate constant decreased to.22 ± 0.07 and 2.18 ± 0.46 s−1 for G3-Chit/ITO and G4-Chit/ITO,espectively, indicating much faster kinetics. Surprisingly, thenfluence of the laser irradiation dose on the electrochemicaleaction rate was rather insignificant being more perceptible for

4-Chit/ITO. This fact indicates that thickness of the film was less

mportant for the electrochemical reaction in this case but moremportant was the exposed area of uncovered graphene to theolution.

ig. 8. Complex plane EIS spectra at ITO (empty circles), untreated G-Chit/ITO (dark yellowraphene load at different potentials in 0.1 mol L−1 KCl. Numbers indicate frequency at m

Acta 132 (2014) 265–276

3.2.2. Electrochemical impedance spectroscopyFig. 8 presents the complex plane impedance spectra at vari-

ous potentials at differently modified electrodes before and afterthe laser irradiation. The spectra from 0.00 to 1.00 V, in the double-layer region, were similar; therefore, only one spectrum at 0.00 Vis shown. As seen, the charge separation was dominating in thedouble-layer region and all spectra at the differently modified elec-trodes were similar. In all cases, the impedance values rose afterthe laser irradiation, especially clearly visible at the low grapheneloads. Further, going to 1.00 V, formation of some surface oxy-species took place; hence, the shape of the spectra at the laserirradiated electrodes was different in the low frequency region (notshown).

As expected, the spectra showed more resistive behaviour of theelectrode in negative potential region, where reduction of ITO orgraphene-oxy-species occurred. G3-Chit/ITO and G4-Chit/ITO hadrather similar shape of the spectra to the ITO containing semicir-cle meaning that charge transfer was dominating at this potential,i.e. the electrochemical reaction occurred. In this case, the semicir-

cle appeared due to reduction of the superficial oxy-species [48].However, no clear semicircles were found at the low graphene loadsdue to significantly lower concentration of oxy-species. At -0.75 V,the reduction of the oxy-species still took place and in all cases

empty symbols), laser treated G-Chit/ITO (coloured filled symbols) with a differentaximum–Z“value. Mean laser power 50 mW.

himica

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R. Celiesiute et al. / Electroc

he clear semicircles were obtained. Also, the impedance valuesncreased in the negative potential region after the laser treatmentue to partial reduction of modified graphene during the laser irra-iation [10,11]. Surprisingly, the highest impedance values werebserved when the graphene load was 1 �g mL−1. Probably, thislm composition had an optimal components ratio and the modi-ed graphene reduction was fastest under these conditions, whilet the lower concentration chitosan slowed down this process andt the higher graphene loads less oxy-species were reduced duringhe laser irradiation.

The spectra were analysed by fitting them to an electrical equiv-lent circuit. Almost all spectra, except the double-layer region,ere analysed as usual for the graphene modified electrodes [49],

pplying the modified Randles’ equivalent circuit consisting of theell resistance, R�, in series with the parallel combination of theonstant phase element, CPE, and the charge transfer resistance,ct, and finally the open Warburg element, Wo, as a specific electro-hemical element describing diffusion [49,50]. CPE was modelleds a non-ideal capacitor:

PE = −1/(Cdliw)a, (4)

where Cdl means the double-layer capacitance; ω is frequency;nd the exponent shows the surface roughness. The spectrat the double-layer region were analysed using all elements ineries:–R�–CPEdl–Wo–[51]. The data of the spectra analysis areresented in Table 2. Errors for all parameters obtained were lowerhan 3%. The diffusion resistance, Rdif, and the time constant, �, wereaken from Wo, which model is described elsewhere [50].

The cell resistance, R�, usually depended on the electrodeomposition; thus, it followed the sequence ITO < G1-Chit/ITO < G2-hit/ITO < G3-Chit/ITO < G4-Chit/ITO, changing from ∼46 � cm−2

o ∼100 � cm−2, respectively. The laser treatment increased R� in

able 2arameters obtained from analysis applying electrical equivalent circuit models of elecFig. 8). Laser scanning speed was 300 mm s−1 during the irradiation.

Electrode Mean laser power, mW E, V Rct, � cm2

ITO 0 -0.75 5.21

-0.50 16.1

0.00 -

G1- 0 -0.75 5.31

Chit/ITO -0.50 14.5

0.00 -

50 -0.75 3.10

-0.50 16.0

0.00 -

G2- 0 -0.75 5.52

Chit/ITO -0.50 11.5

0.00 -

50 -0.75 6.35

-0.50 59.4

0.00 -

G3- 0 -0.75 6.18

Chit/ITO -0.50 10.3

0.00 -

50 -0.75 5.12

-0.50 9.86

0.00 -

200 -0.75 3.92

-0.50 5.44

0.00 -

G4- 0 -0.75 6.86

Chit/ITO -0.50 11.5

0.00 -

50 -0.75 11.2

-0.50 18.1

0.00 -

200 -0.75 2.98

-0.50 4.16

0.00 -

Acta 132 (2014) 265–276 273

∼10% due to a formation of some organic products from chitosanduring the laser irradiation.

Comparison of the EIS parameter is presented in Table 2. As seen,the slight changes were observed in Rct after the ITO modificationwith G-Chit, but it increased after the laser treatment again due tothe laser-induced changes in the chitosan net. Cdl also increasedafter the electrode modification and especially after the laser treat-ment. This increase also depended on the graphene load, as seenfrom Fig. 9: The capacitance increased with the graphene amount inthe film; as well as a significant increase in capacitance at differentpotentials was observed after the laser treatment (Table 2). Com-paring dependence of the capacitance on the laser irradiation dosesat the same graphene load almost a linear dependence was obtained(Table 2). This fact suggests that the laser treatment improves elec-trochemical properties of the G-Chit modified electrode due tocreation of the edge defects and formation of nanocrystalline struc-tures from graphene flakes after the laser irradiation as discussedin Section 3.1.1.

Diffusion parameters show the increase of Rdiff and � after theITO modification and it was more related to the chitosan film thick-ness than to the graphene activation: It took longer for electroactivespecies to penetrate the film. Hence, � at all electrodes was lowerin the double-layer region, where the surface was less smooth(according to values), showing that diffusion was faster than atthe negative potentials, where reduction of oxy-species occurred.

The imaginary part, Z“, as a function of frequency, presented inFig. 10, represents a powerful way to evaluate the time constantsof interfacial processes, since the peaks in the plots correspondto the situation when ωRC = 1 for situations where charge sep-

aration (capacitance, C) and charge transfer (resistance, R) are inparallel, with ω the frequency of the voltage perturbation in radi-ans [51]. As seen, the peaks moved to lower frequency values inthe plots at -0.5 V (Fig. 10A,B) after the laser irradiation, mainly

trochemical impedance spectra registered at differently modified ITO electrodes

Cdl, �F cm−2 sn-1 � Rdif (Wo), � cm2 � (Wo), ms

19.5 0.983 35.0 4.9114.8 0.982 33.8 3.4535.8 0.889 29.9 0.5222.8 0.923 10.8 28.318.8 1.000 28.4 22.015.0 0.928 45.1 2.8931.6 0.999 31.6 10.332.4 1.005 27.5 4.2734.2 0.989 28.0 1.5132.3 0.925 11.8 60.526.8 1.000 35.9 21.018.2 0.935 50.9 1.9431.6 0.979 71.4 5.1232.4 1.008 57.9 3.7434.2 0.989 58.5 1.6236.0 0.870 30.1 6.0925.8 0.894 33.0 4.8050.9 0.861 18.7 0.4633.0 0.932 68.4 17.925.3 0.944 21.7 2.1758.7 0.862 47.3 0.87197 0.864 106 25.4193 0.873 114 6.34125 1.004 72.3 3.1152.6 0.838 32.2 6.1939.4 0.864 35.8 6.0629.3 0.881 30.8 0.4367.2 0.894 42.6 14.331.8 0.866 13.3 3.1564.9 0.863 20.9 0.63210 0.918 30.9 10.1213 0.913 5.18 2.04173 0.844 19.4 1.47

274 R. Celiesiute et al. / Electrochimica Acta 132 (2014) 265–276

Fig. 9. Dependence of capacitance calculated from EIS data on graphene load(A), where empty symbols mean untreated samples and filled symbols indicatepicosecond-laser treated samples at 0 V (circles) and 0.75 V (squares); and on laserp

dfiIeAcCf

cZt

de-dt-alm

Fig. 10. EIS,Z“vs. f, spectra as dependence on the graphene load (A,B) and the meanlaser power at G4-Chit/ITO (C). ITO (empty circles), untreated G-Chit/ITO (dark yel-

The effect of laser irradiation on the chitosan-modified graphenecast on the ITO electrode surface from the aqueous chitosan sus-

ower (B). G3-Chit/ITO sample is presented in (B). All other conditions as in Fig. 8.

ue to the reduction of oxy-species, which made charge trans-er more complicated. On the other hand, the peaks decreasedn their height showing changes of the surface at the interface.n some cases, when the graphene load was lower, the peakven shifted out of scale showing the very slow charge transfer.t this potential, the time constant at the laser treated samplesan be described by the following sequence: G1-Chit/ITO < G2-hit/ITO < G3-Chit/ITO < G4-Chit/ITO starting with values below

< -1 and ending with f = 0.004.At the positive potentials, no peaks were observed because the

harge separation was predominant but clear shift of the rise of“values to more negative direction was observed in all cases afterhe laser treatment (Fig. 10B).

The same plots were drawn in order to investigate the depen-ence of the time constant on the laser irradiation dose. As anxample, results for the G4-Chit/ITO electrode are presented at both0.5 and 0.75 V (Fig. 10C). As seen, the time constant was indepen-ent on the mean laser power applied up to 150 mW. For the samplereated with 200 mW, it shifted to more negative values, i.e. from0.5 to -0.7, but peak height decreased with the laser power appliedlmost linearly, showing changes caused at the surface during the

aser irradiation, which was also observed investigating the surface

orphology with SEM (Fig. 6) and Raman spectroscopy (Figs. 1–4).

low empty symbols), laser treated G-Chit/ITO (coloured filled symbols) at differentpotentials in 0.1 mol L−1 KCl. Graphene loads in (A,B) are given on graphs. The meanlaser power in (A,B) is 50 mW.

4. Conclusions

pension was studied applying spectroscopic and electrochemicalmethods. The graphene load and the laser irradiation dose were

himica

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aried at the electrode surface in order to evaluate their impact onhe resulting film quality.

The Raman spectroscopy showed that the laser irradiationut out the graphene flakes into smaller pieces inducing moredge defects, and the higher graphene load facilitated forma-ion of a larger amount of side defects after the laser treatment.he high laser irradiation dose resulted in the nanocrystalineraphene formation and significant thinning of the graphene-hitosan film. All typical chitosan groups remained after the laserrradiation although the corresponding peak intensity in FTIRpectra decreased. Microscopic investigation showed reduction inhickness of the chitosan layer after the laser irradiation and moreraphene sheet edges were exposed on the electrode surface.

Electrochemical investigation showed diffusion problems of theedox species through the G-Chit film which decreased significantlyfter the laser irradiation together with an electroactive area of thelectrode due to the thinning of G-Chit layer in 200-300 nm. Thishowed blocking of the electroactive centres after the grapheneodification with chitosan while the laser irradiation producedore such centres due to cutting graphene sheets into nanocrys-

alline structures. These structure changes improved significantlyharge transfer diminishing the charge transfer coefficient from0.76 (for G-Chit/ITO) to 0.55 (for the same electrode after the

aser irradiation) and the rate constant from ∼17.5 to 3.2 or 2.2−1, depending on the graphene load.

The laser irradiation, especially with higher mean power than50 mW (irradiation dose 3.34 J cm−2), caused increase in capaci-ance at the electrode surface due to formation of nanocrystallineorm of graphene. The time constant investigation demonstratedhat the charge transfer decreased after the laser irradiation show-ng more complex charge transfer process due to structural changest the electrode surface. Electrochemical investigation of laserreated G-Chit/ITO samples led to suggestion that such compos-te electrodes are promising material to use as a substrate to theensor development.

cknowledgements

This research is funded by the European Social Fund under thelobal Grant measure, Project No. VP1-3.1-SMM-07-K-01-124.

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