Article pubs.acs.org/JPCC
Toward Graphene/Silicon Interface via Controlled Electrochemical Reduction of Graphene Oxide Andrea G. Marrani,† Robertino Zanoni,† Ricardo Schrebler,‡ and Enrique A. Dalchiele*,§ †
Dipartimento di Chimica, Università di Roma La Sapienza, p.le A. Moro 5, I-00185 Rome, Italy Instituto de Química, Facultad de Ciencias, Pontificia Universidad Católica de Valparaíso, Av. Brasil 2950, Valparaíso, Chile § Instituto de Física & CINQUIFIMA, Facultad de Ingeniería, Julio Herrera y Reissig 565, C.C. 30, 11000 Montevideo, Uruguay ‡
ABSTRACT: A novel experimental approach to produce graphene/silicon interface is reported, which consists of electrochemically reduced graphene oxide directly in contact with Si(111). The utilized procedure makes use of differently concentrated graphene oxide solutions drop-casted onto a flat crystalline hydrogenated Si(111) surface. Such modified surface was utilized as a working electrode in an electrochemical cell with aqueous electrolyte and subject to cyclic voltammetry in different conditions. After the electrochemical treatments, the electrodes were characterized by means of field-emission scanning electron microscopy microscopy and Raman and X-ray photoelectron spectroscopies. The overall results demonstrate the transformation of graphene oxide into electrochemically reduced graphene oxide over the silicon surface, as evidenced by the abatement of the spectroscopic features associated with oxidized carbon groups and by the increase in the number of graphene-like sp2 carbon domains. These outcomes are new and a step forward in the direction of an easy procedure to high quality graphene/silicon interfaces.
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In fact, the substantial sp3 fraction in GO renders it an insulating material.24 In the literature, a model of graphene oxide consisting of two different components, one being graphene with a low degree of oxidation while the other an oxidative debris (OD) of graphite on the graphene surface, has been proposed.29−32 This model has been recently questioned by different authors,33,34 and at present this issue is still open to debate. The intrinsic structural and electronics properties of graphene can be partially restored upon reduction of GO, the resulting rGO material usually being considered as one kind of chemically derived graphene.22,27,35,36 By incremental removal of the oxygen-containing groups, a partial recovery of a conjugated structure is achieved, transforming this initial insulator GO material into a semiconductor and ultimately into a graphene-like semimetal.24,27 Reduced graphene oxide (rGO), in which sp2 domains with few nanometers are surrounded by sp3-hybridized carbon atoms, usually behaves as a p-type semiconductor with a large bandgap, whose width can be tuned by controlling the reduction degree of rGO. Such material has been proven to be suitable as a 2D semiconductor for different photonics and optoelectronic devices (chemical/ biological sensors, thin film transistors, solar cells electrocatalysts, photoluminescent devices, field emitters and non-
INTRODUCTION Graphene, a 2D monolayer of sp2-hybridized carbon atoms arranged in a honeycomb lattice structure, and graphene-based materials have attracted research interest due to their unique and excellent electronic, mechanical, thermal, and optical properties as well as their high specific surface areas.1−8 These unique features offer great promise for a wide range of applications in nanoelectronics, sensors, energy storage, and capacitors.6,8−10 To achieve these applications, a large-scale production of high quality graphene sheets in an efficient and effective way is required.6,8,11 In the past years, graphene has been prepared by several approaches, such as micromechanical exfoliation of graphite (Scotch tape method),12−14 chemical vapor deposition,12,15 epitaxial growth,16−18 exfoliation of oxidized graphite, and subsequent reduction of graphene oxide (GO).13,19 The last is a low-cost and easily up-scalable method toward the large scale production of graphene material.2,20,21 The resulting material named as reduced graphene oxide (rGO) exhibits properties between those of graphene and GO.22,23 GO can be viewed as graphene decorated with randomly distributed oxygen-containing functionalities on both sides of the basal plane and edges,19,23−28 i.e., epoxy and hydroxyl functional groups occupying the basal plane and carbonyl, carboxylic acid, and lactol functionalities attached to the edges.12,28 The presence of these moieties breaks the long-range conjugated network and π-electron cloud, leading to a degradation of carrier mobility and conductivity, giving GO its insulating characteristics.24,25,28 © 2017 American Chemical Society
Received: January 23, 2017 Revised: February 23, 2017 Published: February 23, 2017 5675
DOI: 10.1021/acs.jpcc.7b00749 J. Phys. Chem. C 2017, 121, 5675−5683
The Journal of Physical Chemistry C
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volatile devices).5,36−39 Moreover, because of its reasonably reduced number of functionalities, a large number of remaining electroactive sites, and the structural similarity with graphene, rGO exhibits the outstanding merit of high conductivity and chemical/electrochemical activity on the same electrode.25 In consequence, GO and rGO are still considered hot topics in research and development of graphene and graphene-based materials, in particular in regard to mass applications.27 Reducing GO to produce rGO is an extremely vital process because the electronic and physical properties of rGO films depend highly on the efficiency of reducing agents and reduction time; therefore, the variation of such parameters determines how close rGO can get to pristine graphene.22,23 Many different reduction methodologies have been used in an attempt to increase the efficiency of the reduction process and improve the final optoelectronic properties of the rGO.22,23 Three major reduction methods are available: thermal treatment, chemical reduction, and electrochemical reduction.13,23,28,40 Thermal treatment requires high temperatures (>1000 °C) and ultrahigh vacuum in order to achieve highly reduced GO, conditions that are not compatible with most polymer substrates.23,40 In the case of chemical reduction, hazardous reducing chemical agents such as hydrazine are employed.13,40,41 Therefore, a green synthesis of rGO under mild conditions may be preferred.13 In this sense, electrochemical reduction method is one promising green strategy for rGO synthesis.13,40,42−44 Electrochemical reduction of GO is a technique that has been shown to produce very high-quality rGO, almost identical in terms of structure to pristine graphene.23 Moreover, the electrochemical reduction route is a simple, rapid, and efficient method.42 The highly negative potential used in the electrochemical reduction of GO changes the Fermi energy level of the electrode material surface, and thus direct charge transfer efficiently reduces the oxygencontaining functional groups on the electrode surface.40,42 By tuning the cathodic electrochemical potential, the reduction level of the deposited rGO can be achieved (i.e., possibility of in situ tuning of the sp2/sp3 fraction), and consequently one can readily modulate its optoelectronic properties.20,24,28,38 Electrochemically reduced graphene oxide (ERGO) layers have been grown onto different conducting (glassy carbon,41,45,46 Pt,21 Au,47,48 carbon,49 FTO,50 and ITO51) and nonconducting (quartz41) substrates. On the other hand, in recent years, a great deal of interest has been focused on rGO/Si heterojunction solar cells,52 rGO/Si UV-near-infrared photodetectors,38 and rGO/Si Schottky diodes.53−56 Therefore, since an effective integration of rGO and silicon surface is essential to develop these optoelectronic devices, the synthesis of rGO onto silicon surface becomes the keystone step. However, to the best of our knowledge, no work has been reported on the study of growth of ERGO onto a silicon substrate. In the present work, the electrochemical reduction of dropcasted GO layers onto H-terminated n-type Si(111) electrodes in a neutral buffer solution has been studied. In order to better understand the structure−property relationship, different spectroscopical techniques (i.e., Raman and XPS) have been applied to reveal the structural evolution of GO to ERGO upon electrochemical reduction. Morphological characterization of both GO and ERGO layers has also been done.
Article
EXPERIMENTAL SECTION
All the chemicals used here were of analytical reagent grade and were used as received. Doubly distilled water was used. Commercially available graphene oxide (GO) was purchased from Graphenea (Graphenea Headquarters, San Sebastián, Spain) and consists of single-layered graphene oxide dispersed in water at a concentration of 4 mg mL−1. This GO was synthesized using a modified Hummers oxidation method as indicated by the supplier. GO aqueous suspensions with two different working concentrations of 0.004 and 0.06 μg/μL were prepared by diluting the commercial mother solution by distilled water. In all cases, the GO mother solution was initially ultrasonicated for half an hour in order to exfoliate into GO nanosheets and then obtain a higher percentage of GO monolayer flakes. Single-side polished Si(111) wafers (Sieger Consulting), 380 μm thick, n-type (0.005−0.02 Ω cm resistivity), with approximate areas of 1 cm2, were cleaned by sequential immersion for 1 min each in a sonicated bath of CH3OH (Carlo Erba Reagents), acetone (Sigma-Aldrich), trichloroethylene (Carlo Erba Reagents), and acetone. The samples were then dried under a stream of N2(g). After cleaning, the samples were placed directly in 40% NH4F(aq) (Fluka) for 10 min to etch the native oxide layer and produce a H-terminated Si(111) surface. After removal from the etching solution, the samples were rinsed thoroughly with H2O and dried under a stream of N2(g). GO thin layers onto the H-terminated n-type Si(111) surface were prepared as following: 50 μL of GO suspension (in one case 0.004 μg/μL and in the other 0.06 μg/μL), previously ultrasonicated for half an hour, was drop-casted onto the surface of the H-terminated silicon surface using a micropipet and dried in air under heat (50 °C) for 15 min. After this procedure, an ohmic contact was made to the hybrid GO/Si sample, which was scratched on its back, unmodified side, rubbed with Ga−In eutectic, and attached to a copper contact. The working electrode setup was obtained by pressing the GOmodified Si sample against an O-ring, sealing a small aperture in the PTFE electrochemical cell, exactly defining the electrode area (0.3 cm2). All the electrochemical experiences were carried out in a conventional three-electrode PTFE cell: working electrode, counter electrode (platinum wire), and reference electrode (Ag/AgCl(sat.), E = 0.195 V vs NHE). All the potentials in this study are referred to this electrode. As mentioned above, the H-terminated n-type Si(111) substrates modified with GO constituted the working electrodes. Electrochemical experiments were performed in 1 M phosphate (K2HPO4/KH2PO4) buffered solution as supporting electrolyte, with a pH fixed at 7.2. The electrochemical reduction of the GO layer onto the n-Si(111) surface was carried out by cyclic voltammetry within the potential window 0.2 to −1.5 V and varying the potential scan rates from 5 to 100 mV s−1. Electrochemical measurements were performed using an Autolab PGSTAT12 potentiostat/galvanostat driven by the Autolab software Nova 1.10. In order to calculate the density of oxygen-containing groups in graphene oxide, the specific surface area of graphene of 2630 m2 g−1 was used, but this was divided by two to obtain the effective specific surface area (1315 m 2 g −1 ). 45 This computation takes into account that the surface-bound oxygenated groups may be found on both sides of a single plane of graphene.45 Hence, the effective surface area of a 5676
DOI: 10.1021/acs.jpcc.7b00749 J. Phys. Chem. C 2017, 121, 5675−5683
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The Journal of Physical Chemistry C typical sample with a mass of 3 μg is 3.945 × 1015 nm2. The number of oxygenated groups per square nanometer is given by (number of moles of oxygenated groups × Avogadro’s number)/3.945 × 1015 nm2.45 The number of moles of oxygenated groups can be obtained by means of electrochemical measurements (vide infra). Topographic investigation of GO and ERGO deposits was performed using a field-emission scanning electron microscope (FE-SEM) Zeiss Auriga 405 (c/o SNN-Lab-Sapienza Nanoscience & Nanotechnology Lab). Raman spectra were measured at room temperature in backscattering geometry with an inVia Renishaw micro-Raman spectrometer equipped with an air-cooled CCD detector and super-Notch filters. The emission line at 488.0 nm from an Ar+ ion laser was focused on the sample under a Leica DLML microscope using a 5× objective. The power of the incident beam was about 5 mW. Ten 10 s accumulations were generally acquired for each sample. The resolution was 2 cm−1, and spectra were calibrated using the 520.5 cm−1 line of a silicon wafer. La (the average crystallite size) of the sp2 lattice was calculated using the equation57 ⎛ I ⎞−1 La (nm) = (2.4 × 10−10)λlaser 4⎜ D ⎟ ⎝ IG ⎠
Figure 1 shows typical repetitive cyclic voltammograms (10 cycles) for a GO modified H-terminated n-type Si(111)
Figure 1. Cyclic voltammograms recorded during the electrochemical reduction of a GO/Si electrode in a pH = 7.2 buffered solution. The GO layer was obtained by drop-casting 50 μL of a 0.06 μg/μL GO aqueous suspension onto the silicon surface. The arrow indicates the potential scan direction. The inset shows the voltammetric response of a bare H-terminated n-type Si(111) electrode in the same electrolyte. Scan rate for both cases: 20 mV s−1.
(1)
electrode (GO/Si) in the neutral phosphate-buffered solution. It can be seen that during the first potential scan toward the cathodic potentials a well-defined irreversible large reduction peak at −1.2 V with a starting potential at −0.52 V appears. On the other hand, no voltammetric waves can be observed on the control bare H-terminated Si(111) electrode (inset Figure 1) in the potential range from 0.0 to −2.0 V. Since the reduction of water to hydrogen occurs at more negative potentials (E < −1.5 V), this large cathodic current can be attributed to the reduction of the surface-bound oxygenated groups, primarily epoxyl and carbonyl groups, in agreement with previously reported works on electrochemical reduction of GO layers onto a glassy carbon electrode.44,58−61 The consecutive second cycle (see Figure 1) does not exhibit any voltammetric peak in that potential scan range, indicating that all the oxygenated groups have been successfully quantitatively reduced during the first potential scan.3,59,62 Additionally, no further reduction has been observed in the consecutive cyclic voltammetry potential scans. As to the color of the graphene film, while GO is a brownish material, after electrochemical reduction, the freshly formed ERGO exhibited a black color. The integrated area of the large cathodic peak depicted in Figure 1 is directly proportional to the amount of reducible surface-oxygenated species of the GO layer. Therefore, by using Faraday’s law (eq 2), it is possible to quantify the electrochemically reducible oxygen-containing groups:
where ID and IG are the intensities of the Raman D and G bands, respectively, and λlaser is the laser wavelength line. Pure graphite samples for XPS were obtained by doubly expholiating with Scotch tape a 1 cm2 sample of graphite (Carbone Lorraine). XPS measurements were carried out using a modified Omicron NanoTechnology MXPS system equipped with a monochromatic Al Kα (hν = 1486.7 eV) X-ray source (Omicron XM-1000), operating the anode at 14−15 kV and 10−20 mA. All the photoionization regions were acquired using an analyzer pass energy of 20 eV, and takeoff angles of 11° with respect to the sample surface normal have been adopted. The experimental spectra were theoretically reconstructed by fitting the secondary electrons background to a Shirley function and the elastic peaks to pseudo-Voigt functions described by a common set of parameters (position, fwhm, Gaussian− Lorentzian ratio) free to vary within narrow limits. Accuracy of binding energy values was ±0.05 eV, while error associated with quantitation was ±10%.
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RESULTS AND DISCUSSION The preparation of a n-Si(111) surface covered with electrochemically reduced graphene oxide (ERGO/Si) was carried out by the two-step electrochemical approach: i.e., first, a thin film of GO was deposited onto the H-terminated Si(111) surface by drop-casting and subsequently dried out to form a GO modified Si electrode (GO/Si); second, the GO-coated silicon electrode was subjected to electrochemical reduction. Hydrogen evolution is a competing reaction for GO electrochemical reduction in aqueous electrolyte. Therefore, the cathodic potential limit has been selected in order to ensure the reduction of the more stable surface-oxygenated groups (which required hard reduction conditions) and at the same time to limit the hydrogen evolution reduction. In fact, the choice of cathodic potentials below −1.5 V vs Ag/AgCl would result in the evolution of hydrogen bubbles, which can lead to the peeling off of the ERGO film from the surface of the silicon electrode.43,47
n = Q /zF
(2)
where n corresponds to the number of moles of oxygencontaining groups, Q is the integrated charge corresponding to the large cathodic peak, F is the Faraday constant, and z is the number of exchanged electrons per one surface-bond group. In a general way, the accepted reaction governing the electrochemical reduction process of the surface-oxygenated groups onto the GO surface can be expressed as follows:6,13,45,63 GO + aH+ + be− → ERGO + c H 2O
(3) 64
This latter is a solid-to-solid electrochemical reaction, where for the case of epoxyl, carbonyl, peroxyl, and aldehyde 5677
DOI: 10.1021/acs.jpcc.7b00749 J. Phys. Chem. C 2017, 121, 5675−5683
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The Journal of Physical Chemistry C
0.004 μg/μL, respectively, has been calculated from the concentration nominal value declared by the supplier (being the area 0.3 cm2 and assuming the density of GO is close to 1.8 g cm−3 65). Moreover, on the basis of the predicted effective single sheet thickness of GO around 1.4 nm (due to the Hbonded water molecules and intrinsic structural defects),48,66,67 the obtained drop-casted GO films are estimated to consist of approximately 40 and 3 layers of individual GO sheets, if the above-reported GO suspensions are considered, respectively. Figure 3 depicts the voltammograms of GO modified silicon electrodes with two different GO amounts, i.e., GO drop-casted
oxygenated groups two electrons are involved in the reduction process, corresponding to a z = 2 (b = 2).45 Furthermore, taking the n value calculated through eq 2, and considering the amount of graphene oxide deposited onto the silicon electrode surface (see Experimental Section), an average value of oxygencontaining groups of ∼5 groups per nm2 of the graphene sheet surface area in the initial graphene oxide have been calculated. This is very close to the value of 4.30 groups per square nanometer reported by Pumera et al.60 for a GO synthesized through the Staudenmaier method. The electrochemical features appearing during the CV reduction of oxygen-containing groups in GO should mainly depend on the scan rate, pH, and GO mass electrode (GO thickness).41,50,51 Different CV scan rates have been tested for the electrochemical reduction of the GO layer onto the silicon substrate. Figure 2 shows cyclic voltammograms recorded
Figure 3. Cyclic voltammograms recorded during the electrochemical reduction of a GO/Si electrode with two different GO masses, i.e., GO layer drop-casted from a 0.004 μg/μL (black curve) and 0.06 μg/μL (red curve) GO aqueous suspension, in a pH = 7.2 buffered solution. Scan rate: 20 mV s−1. Inset: magnified view of the cathodic peak in black curve.
Figure 2. Cyclic voltammograms recorded during the electrochemical reduction of a GO/Si electrode in a pH = 7.2 buffered solution at different scan rates: 5 mV s−1 (black curve), 10 mV s−1 (red curve), 20 mV s−1 (green curve), 50 mV s−1 (cyan curve), and 100 mV s−1 (blue curve). The GO was layer obtained by drop-casting 50 μL of a 0.06 μg/μL GO aqueous suspension onto the silicon surface. The arrow indicates the potential scan direction.
layer from a 0.004 and 0.06 μg/μL GO aqueous suspension, at a scan rate of 20 mV s−1. In the former case, a cathodic peak can be appreciated at −0.95 V. In the latter case, instead, an increase of deposited GO results in a corresponding increase of the cathodic peak current and in a negative shift of the reduction peak position to −1.08 V. These findings are in agreement with similar studies on GO modified glassy carbon electrodes.61 As has been said above, the integrated area of these large cathodic peaks is directly proportional to the amount of reducible oxygen-containing groups. By evaluating the integrated area ratio between the larger cathodic peak and the smaller one, a value of ∼15 has been obtained, which agrees very well with the ratio between the associated GO suspension concentrations (0.06 μg/μL:0.004 μg/μL = 15). This indicates a quantitative reduction of electrochemically reducible oxygencontaining groups on GO surface. Figure 4 shows FE-SEM micrographs of GO and ERGO layers grown onto an hydrogen-terminated n-type Si(111) surface (GO/Si and ERGO/Si, respectively). Figures 4a and 4b show micrographs of GO/Si samples obtained by drop-casting from GO aqueous suspensions of two different concentrations: 0.004 and 0.06 μg/μL, respectively, whereas Figures 4c and 4d are related to ERGO/Si samples. These latter have been obtained by cyclic voltammetry (at a scan rate of 20 mV s−1 in pH = 7.2 buffered solution), from GO/Si samples obtained from 0.004 μg/μL (Figure 4c) and 0.06 μg/μL (Figure 4d) concentrated GO aqueous suspensions. The FE-SEM image depicted in (a) reveals a continuous film with a random distribution of wrinkles, a morphology which evolves into a
during the electrochemical reduction of a GO/Si electrode (GO drop-casted layer from a 0.06 μg/μL GO aqueous suspension), at different scan rates from 5 to 100 mV s−1, as indicated. During the cathodic sweep, at low scan rate values, two very well resolved electrochemical features can be appreciated. On the other hand, as the scan rate increases, a large peak can be seen, likely arising from the contribution of three or more irreversible electrochemical processes. Moreover, as the scan rate increases, the peak potential of these electrochemical features shifts to more cathodic values, and the corresponding peak intensity increases. This behavior is in a general way similar to that reported for the electrochemical reduction of a GO layer onto a FTO/glass substrate in an organic solvent,50 but in that case, the reported voltammogram is quite featureless, with only one large peak.50 To further gain insight into the electrochemical reduction process of GO layers onto a silicon substrate, the effect of the GO mass (i.e., number of layers) on the electrochemical response has been investigated. In the drop-casting technique, the thickness of the GO film can roughly be controlled by modulating the GO dispersion concentration and the aliquot volume.48 The number of overlaying ERGO sheets can be estimated from the thickness of the precursor GO film. An estimated thickness of the GO layer around 56 and 4 nm for drop-casted GO suspensions with concentrations of 0.06 and 5678
DOI: 10.1021/acs.jpcc.7b00749 J. Phys. Chem. C 2017, 121, 5675−5683
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The Journal of Physical Chemistry C
Figure 4. FE-SEM micrograph images of GO/Si samples obtained by drop-casting 50 μL of (a) 0.004 μg/μL and (b) 0.06 μg/μL GO aqueous suspensions onto the silicon surface and the corresponding ERGO/Si samples (c and d) obtained after application of cyclic voltammetry (see text for details) to the samples reported in micrographs (a) and (b), respectively.
sp2-bonded carbon atoms and representing the relative degree of graphitization. The D band is related to the defect breathing mode of κ-point phonons of A1g symmetry68,72,73 and, together with 2D band, is also associated with the vibrations of sp3 carbon atoms in disordered graphene (a second-order process attributed to local defects, vacancies, and grain boundaries).71,72 The typical Raman spectra depicted in Figure 5, as has been said above, exhibit the presence of the usual two prominent D and G bands at ∼1360 and ∼1596−1601 cm−1 for both GO and ERGO thin films. It can be appreciated that the D peak intensity increases with respect to the G peak for the ERGO/Si sample. The intensity ratio of the D and G peaks (ID/IG) has been used as an indicator of disorder in graphene materials (such as arising from ripples, puddles, edges, the presence of domain boundaries, or many other defects), as expressed by the sp2/sp3 carbon ratio.22,46,57,74 In fact, it has been widely used as a measure of the size of the sp2 ring clusters in a network of sp3and sp2-bonded carbons.73 It must be pointed out that while the ID/IG ratio is a measure of the disorder in graphene, it does not reflect the oxidation and reduction degree because the ID/ IG ratio can be influenced by the different defects (see above).69 In the present work, after electrochemical reduction of GO (vide inf ra), we found a higher ID/IG ratio (1.14) for ERGO than for pristine GO (0.70), which implies the formation of new domains of conjugated carbon atoms (bonded in sp2 hybridization) accompanying the removal of the oxygencontaining groups.75 This result supports a well-conducted reduction of GO76 and, on the other hand, suggests that more defects could be introduced during the electrochemical reduction process. Using the relation proposed by Pimenta et al.57 to obtain the lateral dimension of sp2 ring clusters (inplane crystallite size La), eq 1, average graphitic domain sizes of La = 19 nm and La = 12 nm in pristine GO and ERGO, respectively, have been calculated. Moreover, it can be noted in Figure 4 that the GO layer exhibits a G band at 1601 cm−1, while the corresponding G band of ERGO thin film is at 1596
smoother surface upon electrochemical reduction shown in (c). The initial corrugated and folded aspect of the continuous film for the higher GO concentration, shown in (b), becomes upon reduction closer to the ERGO morphology for the lower concentration sample. To gain insight into the structural and electronic properties of both GO and ERGO films, Raman spectroscopy was next performed on the graphene nanostructures.68,69 Raman spectroscopy is a powerful nondestructive tool to distinguish ordered and disordered crystal structures of carbon.62,69−71 Figure 5 shows the Raman spectra of GO/Si and ERGO/Si samples. The typical D and G bands along with weak 2D and D + G peaks have been found in all spectra, in agreement with what reported in the literature.71,72 The G band is usually assigned to the E2g phonon related to the in-plane vibration of
Figure 5. Raman spectra of a pristine GO/Si (red trace) and an ERGO/Si (blue trace) sample. The red graph was upshifted for the sake of comparison. The GO/Si sample was obtained by drop-casting 50 μL of a 0.06 μg/μL GO aqueous suspension onto the silicon surface. The ERGO/Si sample was obtained from a GO/Si sample after application of cyclic voltammetry (at a scan rate of 20 mV s−1 in a pH = 7.2 buffered solution). 5679
DOI: 10.1021/acs.jpcc.7b00749 J. Phys. Chem. C 2017, 121, 5675−5683
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The Journal of Physical Chemistry C cm−1. This red-shift of the G band of ERGO can be attributed to the partial recovery of the hexagonal sp2 carbon network in this sample77 and adds further evidence that the electrochemical route to GO reduction is effective.78 On the other hand, it can be noted that the prominent D peak significantly narrows upon electrochemical reduction of GO, with a variation of full width at half-maximum (fwhm) from 160 cm−1 in the GO sample to 72 cm−1 in the ERGO one, which compares very well with values reported for other chemically, ́ electrochemically or thermally reduced GO layers.40 DiezBetriu et al. considered that the fwhm of the Raman D peak can be taken as a parameter to define the degree of reduction of GO.79 According to this, the D peak fwhm of 72 cm−1 found in the present work suggests that only a low fraction of carbon atoms (
