In Situ Raman Studies of Electrically Reduced Graphene Oxide and Its Field-Emission Properties PDF

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Article pubs.acs.org/JPCC In Situ Raman Studies of Electrically Reduced Graphene Oxide and Its Field-Emission Properties Satyaprakash Sahoo,* Geetika Khurana, Sujit K. Barik, S. Dussan, D. Barrionuevo, and Ram S. Katiyar* Department of Physics, University of Puerto Rico, San Juan, Puerto Rico, Unite...

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In Situ Raman Studies of Electrically Reduced Graphene Oxide and Its Field-Emission Properties Sandra Dussan The Journal of Physical Chemistry C

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Article pubs.acs.org/JPCC

In Situ Raman Studies of Electrically Reduced Graphene Oxide and Its Field-Emission Properties Satyaprakash Sahoo,* Geetika Khurana, Sujit K. Barik, S. Dussan, D. Barrionuevo, and Ram S. Katiyar* Department of Physics, University of Puerto Rico, San Juan, Puerto Rico, United States S Supporting Information *

ABSTRACT: Electric-field-dependent in situ Raman studies have been carried out on chemically prepared graphene oxide. The Raman spectra show significant changes with increase in the applied electric field; in particular, the intensity of the G peak decreases with electric field. This behavior is typical for chemically or thermally reduced graphene oxide. To understand the nature of reduction, we compared the temperaturedependent and electric-field-dependent Raman spectra of graphene oxide and found that the evolutions of Raman spectra are not in agreement with each other, except the intensity of the G peak that decreases in both cases. The D peak broadens significantly with increase in temperature, whereas it sharpens in the case of applied electric field. The electronfield-emission properties of the electrically reduced graphene oxide were also carried out, and the turn-on field was found to be 9.1 V/μm.

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tion process in GO. In this work, we report in situ electric field dependent Raman studies of GO and try to understand the underlying reduction mechanism by comparing the electrical dependent Raman spectra with that of a temperaturedependent Raman spectra of GO. We also studied the fieldemission properties of electrically reduced GO. GO synthesis was performed using a modified Hummers method.20 Concentrated H2SO4 (50 mL) was added to highly oriented pyrolytic graphite (HOPG) in a round-bottomed flask at room temperature. The mixture was continuously stirred with the help of a Teflon-coated magnetic stirrer. The flask was then placed in an ice bath to maintain a low reaction temperature (∼0 °C). Potassium permanganate (KMnO4, 7g) was added very slowly to the solution while maintaining low temperature. The mixture was stirred continuously for 2 h. An excess of distilled water was then added to the solution. Hydrogen peroxide (H2O2, 30 wt %) was added slowly while stirring until the gas evolution stopped. The resultant mixture was stirred for another 15 min and then filtered with the help of a glass filter. Finally, the obtained precipitates were dried for 24 h in a vacuum oven at room temperature. The resultant product was crushed in a mortar pestle, producing a brown powder of GO. A colloidal suspension of GO (1 g with a concentration of 3 g L−1) was prepared in distilled water. In the present study, the exfoliation of the GO sheets was performed by sonicating the graphite oxide in water for 2 h. Raman measurements of the sample were carried out using a Horiba-Yobin T64000 micro-

raphene is the single layer of carbon atoms that exhibits very peculiar physical properties over all other allotropes of carbon, and hence it has gained tremendous research interest.1−6 The charge carriers in graphene are massless Dirac fermions and thus possess very high carrier mobility.4 Since the discovery of graphene, conventional mechanical exploitation of highly oriented pyrolitic graphite remains a potential method of obtaining high-quality graphene.7,8 Recently, the chemical vapor deposition method also showed promise for synthesis of high-quality graphene on the large scale.9 Chemical exploitation of bulk graphite using strong oxidizing agent (Hummers process), which results in large scale graphene oxide (GO), is quite useful for the synthesis of large scale graphene.10 GO shares the similar single and few atomic layer structure of carbon as monolayer or few layer graphene, respectively, but unlike graphene (which has otherwise high electrical conductivity) the presence of hydroxide, epoxy groups in GO makes it highly electrical resistive.11−13 This is undesirable for any electronic applications. There are several ways one can reduce the GO to improve its conductivity. Reductions under strong reducing agent such as hydrazine or annealing at high temperature under different gas environments are the usual methods of reducing GO.13−17 Recently, it has been found that under the influence of electric field, reduced graphene oxide (rGO) can be obtained from GO.18 The latter is a clean way of obtaining r-GO, although the mechanism of the process involved is not well understood. Reduction of GO under electric field could helpful in understanding the resistive switching behavior of GO.19 Raman spectroscopy is a nondestructive technique to understand electron−phonon coupling in graphene. In view of this, it might useful in understanding the mechanism of electric-field-induced reduc© 2013 American Chemical Society

Received: January 17, 2013 Revised: February 20, 2013 Published: February 22, 2013 5485

dx.doi.org/10.1021/jp400573w | J. Phys. Chem. C 2013, 117, 5485−5491

The Journal of Physical Chemistry C

Article

GO layers were placed by dip coating. A typical FESEM image of a few layer GO on the fingers of the Pt IDE is shown in Figure 2a. During the Raman measurements the laser was

Raman system, and a 514.5 nm line of an argon ion laser was used as an excitation source. The morphological characterization was carried out using a field-emission scanning electron microscope (FESEM, JEOL JSM-7500F SEM). The I−V measurements were performed by a Keithley meter (2401A). Thickness and roughness of the GO were measured using atomic force microscopy (AFM) and piezo force microscopy (PFM), respectively (Veeco). Raman spectroscopy is one of the most powerful nondestructive techniques to study carbon materials.21−25 In the past decade, graphene has been studied more extensively using Raman spectroscopy to identify the number of graphene layers and the ratio of sp2/sp3 bonding, monitor the doping concentration, electron−phonon interaction, and so on. Figure 1 shows the comparison of Raman spectra of a monolayer

Figure 2. (a) FESEM image of GO on an interdigital Pt electrode. (b) Schematic diagram of the GO layer and Pt IDE system with applied voltage and focused laser beam for in situ Raman study.

Figure 1. Comparison of Raman spectra of monolayer graphene and graphene oxide.

focused on the section of the GO placed between two fingers of the electrode. Note that the separation between two consecutive metal fingers is ∼30 μm and our laser spot is ∼1.5 μm in diameter. A schematic representation of the GO layer and the Pt IDE system with applied voltage and focused laser beam is shown in Figure 2b. To get the thickness of the GO sample, we performed tapping-mode AFM. The tapping mode AFM image is shown in Figure 3. The thickness of the GO sample varies from 2 to 5 nm. This could be due to the folding of the GO layers. The electric-field-dependent Raman spectra are shown in Figure 4. From Figure 4, one can see that irrespective of applied voltage there are two distinct Raman bands, D and G, at about 1350 and 1580 cm−1, respectively, that are observed. As previously mentioned, the peak at 1350 cm−1 is associated with the sp3 bonding in carbon and is not found in pure graphite; however, the presence of disorder in graphite results in sp3 bonding, which results in the appearance of this peak in graphite and GO. One can see from Figure 4 that the intensity of the G peak with respect to the D peak gradually decreases with an increase in applied voltage. The G peak intensity was significantly decreased as compared with the D peak for the applied voltage of 8 V. Note that in the case of pure GO the G peak is slightly more intense than the D peak. As previously stated the D peak in graphene is associated with defects. It is clear that with an increase in applied voltage the intensity of D peak increases, which strongly indicates that defects are being created with applied voltage. The plot of the intensity ratio of the D peak (ID) to the G peak (IG) is shown in Figure 4b, and it can be seen that the intensity ratio increases with increase in applied voltage; however, a significant change was observed after 6 V. It is also found that the full width at half-maximum (fwhm) of the D peak remains more or less

graphene obtained by micromechanical exfoliation of HOPG with that of chemically obtained GO. Two distinct and sharp peaks can be seen in the case of monolayer graphene at about 1580 and 2780 cm−1, which are the G and 2D Raman peak of graphene. The 2D peak is associated with the two-phonon scattering process whose intensity should be much less as compared with the G peak of the bulk graphene. However, because of resonant conditions in the monolayer graphene its intensity increases significantly over the G peak. The Raman spectra of GO showed two distinct and broad peaks at 1350 and 1580 cm−1. The peak at 1350 is usually associated with defects and is a measure of the sp3 bonding in graphene.22 Note that there is no D peak present in the monolayer graphene, indicating a defect-free sample.22 The G peak is quite broad and blue-shifted as compared with that of G peak of monolayer graphene. The broadening is due to breakdown of the continuous hexagonal honeycomb crystal lattice of graphene in GO. The most striking feature is the significant reduction of 2D band intensity in GO over pure monolayer graphene. It has often been observed that introduction of defects in graphene results in the breakdown of regular hexagonal crystal symmetry and this hinders the condition of resonance and hence reduction of 2D Raman intensity. GO is associated with large defects, mainly due to hydroxyl, and epoxy groups, and these functional defects account for the poor 2D peak in GO. Recently, there have been few reports on the reduction of GO under electric field;18 however, the exact mechanism of the reduction process is not clear. We try to understand the mechanism by recording the in situ Raman spectra of GO under different applied voltages. Different DC voltages were applied across a Pt Inter digital electrode (Pt IDE) on which 5486

dx.doi.org/10.1021/jp400573w | J. Phys. Chem. C 2013, 117, 5485−5491

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Figure 3. Tapping mode AFM image of few layers GO. The height profile is also shown.

distinct. This can be further supported by the fact that at 8 V the G peak splits into two peaks G and D′ at 1578 and 1612 cm−1, respectively, as shown in Figure 5, which indicates that at

Figure 5. Fitted Raman spectra of GO treated with a applied voltage of 8 V.

8 V the GO is reduced and exfoliated heavily by releasing a large number of water molecules. The D′ band is an intravalley second-order Raman scattering process.22 It has been welldocumented that in the case of reduced GO (GO subjected to reduction under hydrazine or thermally reduced) the intensity of the D peak increases as compared with that of the G peak.26−28 In other words, reduction of GO results in the breakdown of carbon−carbon bonds in GO. The Raman spectrum of the post-electric-field-treated sample is very similar to that of chemically (hydrazine) reduced GO (see Figure 6). One can see from the Figure that these spectra are very similar to each other; the intensity ratio of the D peak to the G peak is almost same in both cases. More importantly, there is no shifting of the D and G peak positions in both cases. Hence, at this point, we can claim that the GO can be reduced electrically.

Figure 4. (a) Raman spectra of GO with different applied voltages. (b) Variation of intensity ratio of D to G peak with voltage. Solid curve is guide to the eye.

unchanged until the applied voltage reaches 3 V; however, it starts decreasing above 6 V and becomes considerably narrower at 8 V. This could be due to the excess release of hydroxide groups from GO that reduces the van der Waals interactions between GO inter layers and makes the layer structures more 5487

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change significantly with increase in applied voltage (3.49 and 3.41 nm for 0 and 8 V, respectively). As previously stated, to further confirm the electrical reduction of GO, we studied conductivity of the GO sample as a function of applied voltage. It is well-reported that the GO has lower conductivity as compared with rGO.12 Figure 9a shows I−V characteristic of a GO film prepared identically as prepared for in situ Raman studies discussed previously. As can be seen from the Figure, for pure GO the I−V curve is nonlinear, and the maximum current is 2 nA for the applied voltage of 1 V. This low value of current suggests that GO is highly resistive. Note that the observed I−V trend is consistent with other reported results. We applied different higher DC voltages to the thin film for a certain period of time. To avoid any confusion, hereafter we will refer to the applied higher voltage as treated voltage. After that, the treated voltage was turned off and the I−V measurements were performed by scanning voltage from −1 to 1 V, which is shown in Figure 9a. It was found that the current increases systematically with increases in treated voltage and it reaches milliamperes for treated voltage of 8 V and the I−V characteristic becomes linear, which is ohmic in nature. This result suggests that the electric field can reduce the GO. We recorded Raman spectra after each I−V measurement, and Figure 9b compares the Raman spectra of the GO film (bottom spectra) and the film treated with a voltage of 8 V (top spectra). It can be seen from the Figure that the Raman spectra of the 8 V treated film completely resemble to that of the reduced GO. This result independently is in agreements with the in situ Raman study. We also carried out several similar experiments by varying the thickness of the GO film and found that with increasing thickness of the sample higher voltage is required to reduce it. To understand the uniformity of the reduction over the surface of the rGO film we carried out Raman mapping over a rage of 10 × 10 μm on the rGO film treated with different voltages also at different points on the sample. From the Raman mapping it was found that the GO film was uniformly reduced. Figure 10 shows a typical Raman intensity mappings for the D and the G peak with treated voltage of 6 V. We also measured Raman scattering at several regions on the samples to conform the uniformity of the reduction process. It can be seen from the Figure that the sample is uniformly reduced throughout the entire mapping surface. A graphene paper is shown in Figure 11, which was reduced electrically by applying a DC voltage of 25 V. The GO paper was prepared by filtering the GO solution through a vacuum filter, and it was allowed to dry for 24 h in a vacuum oven at 60 °C. The thickness of the GO paper was found to be 20 μm. When this piece of rGO paper is connected in a electrical circuit, current starts flowing through it and the diode in the circuit glows. However, the diode does not glow if the rGO is replaced by a pure GO. This has been demonstrated in a short movie (see the Supporting Information). We studied the electron-field-emission property of the rGO paper (by taking a small portion of it) in a vacuum chamber. Figure 12 shows the graphs between the applied voltage and the emission current for the electrically reduced GO paper. In our measurements, current was collected on the anode, a molybdenum rod, having 3 mm of diameter, which was located perpendicular to the sample stage, the cathode. Figure 12 shows the electron-fieldemission plot of rGO. We applied the voltage starting from 0.03 kV to the maximum applied voltage of 2.5 kV. The rGO has the turn-on field of 9.1 V/μm. It was also found that as we

Figure 6. Comparison of Raman spectra of electrically reduced GO and chemically reduced GO.

We will verify our claim later by investigating the electrical conductivity of our samples. A discussion on the nature of the reduction process is in order. The reduction could be either thermal or electrical in nature. It has been reported that GO can be reduced by annealing it at high temperature. In our case, heating could take place during the flow of electric current in the graphene sample. To verify the heating effect, we perform a temperaturedependent Raman study of GO sample. Figure 7 shows the

Figure 7. Temperature-dependent Raman spectra of GO.

Raman spectra of GO with increase in temperature. One can see from the figure that with an increase in temperature, fwhm of the D peak increases. If one compares this result with that of the Raman spectra obtained by varying electric field, then it can be seen that the fwhm of D peak narrows down by varying the applied electric field. The intensity of the D peak increases over the G peak in both cases (i.e., in both temperature- and electricfield-dependent Raman studies). This result confirms that the reduction of GO by electric field is not thermal in nature. Hence, the possibility of pure electrical reduction is obvious. A possible mechanism of electrical reduction is explained below. The flow of the current can cause reduction/oxidation of noncarbon species present in the film (their electrolysis) and hence the chemical change of the sample. In the electrolysis process the H+ arise from the ionization of the carboxyl groups of the GO, and the electrons undergo the following reaction: GO + H+ + e¯ → rGO + H2O, and this results in the reduction of GO. We further investigate the PFM studies of GO by applying different voltages (0−8 V). Figure 8 shows the PFM images of the GO samples with different applied voltage. These results show that the roughness of our GO samples did not 5488

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Figure 8. (a−d) PFM image of GO treated with 0, 4, 6, and 8 V, respectively.

Figure 9. (a) I−V characteristics of GO film after application of different treated voltages. (b) Comparison of Raman spectra of GO film when no voltage was applied (bottom spectra) and after a treated voltage of 8 V was applied (top spectra).

Figure 10. Raman intensity mapping (a) for the D peak and (b) for the G peak.

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has the advantage over reduced GO as field emitter because of its sharp tip. Calderon-Colon et al. have studied the field emission in CNT and observed very high current density.39 The field emission property of graphene can be affected by several factors such as thickness, orientation and dimension. In a recent study, Yamaguchi et al.34 have observed a very low threshold field emission from edges of single-layer rGO. The edges with C−O−C bond and similar electronic structure act as a series of emission sites that result in coherent electron beams. In our case, large dimension and high thickness could restrict the low threshold field emission. In contrast, CNT has very confined edges and the C−C bond at the edges preferably causes emission of coherent electron beams and thus high threshold field emission. In...