Pulsed laser assisted reduction of graphene oxide

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Pulsed laser assisted reduction of graphene oxide Lei Huang *, Yang Liu, Le-Chun Ji, Yi-Qun Xie, Tao Wang, Wang-Zhou Shi Key Laboratory of Optoelectronic Materials and Device, Department of Physics, Shanghai Normal University, 100 Guilin Road, Shanghai 200234, China

A R T I C L E I N F O

A B S T R A C T

Article history:

We report a simple approach to reduce graphene oxide (GO) solution by pulsed laser irra-

Received 11 October 2010

diation. The reduction was rapidly carried out at room temperature in only 5 min. The

Accepted 28 January 2011

reduced graphene oxide (r-GO) was characterized with UV–visible spectroscopy, X-ray dif-

Available online 19 February 2011

fraction, Raman spectroscopy, X-ray photoelectron spectroscopy, thermo-gravimetric analysis, Fourier transform infrared spectroscopy, scanning electron microscopy and atomic force microscopy. Based on this reducing method, an r-GO conductive film with a sheet resistance of 53.8 kX/sq was obtained. The pulsed laser reduction of GO in solution creates a new way to produce graphene composites for a variety of applications.  2011 Elsevier Ltd. All rights reserved.

1.

Introduction

Graphene, a single atomic monolayer of sp2-bonded hexagonal carbon with unique electrical, thermal, mechanical and optical properties, has become a promising candidate for next generation electronic materials such as transparent conducting electrodes, sensors, ultracapacitors and others [1,2]. The preparation of high quality graphene on a large scale and in a cost-effective manner is essential for many applications. To date, graphene oxide (GO) has emerged as a precursor for bulk production of graphene-based materials, as it can be synthesized from inexpensive graphite powders [3]. Furthermore, the GO can be reduced to form chemically modified graphene, in which a large portion of oxygen-containing functional groups are removed by low temperature chemical reaction with reducing agents for an extended period of 24 h or more [4–6] or by thermal treatment at high temperatures (>1000 C) [7,8]. Unfortunately, the preferred reducing agent is toxic and the deoxygenating processes are time-consuming and complicated [2–4,9–10]. Most recently, several groups have demonstrated that GO can be reduced by photo-irradiating process, such as UV-induced photocatalytic reduction [11,12], photo-thermal reduction using a pulsed Xenon flash [13], selective reduction by direct laser writing [14]

and laser converted graphene from GO [15]. The advantage of these methods is that photo-irradiating processes do not rely on the use of chemicals or high temperature; especially, shorten the reaction time from several hours to a few minutes. In this paper, a simple method is described for reduction of GO solution by pulsed laser irradiation. The majority of oxygen-containing functional groups of GO were removed by the pulsed laser irradiation. Although GO is an insulator, the reduced graphene oxide (r-GO) produced by this method is electrically conducting. This method provides an effective way to produce graphene composites by premixing with other materials.

2.

Experimental

GO was synthesized from graphite powder by a modified Hummers method [16,17]. A homogeneous GO aqueous suspension (0.1 mg/mL) was achieved by dispersing purified GO in distilled water with the aid of ultrasonication for 1 h. Then, ammonia (A.R.) was added into the above GO suspension to present in solution at pH 9. Afterward, 15 mL of the yellow– brown GO suspension was loaded into a quartz tube for the pulsed laser reduction experiment. The pulsed laser

* Corresponding author: Fax: +86 21 6432 8968. E-mail address: [email protected] (L. Huang). 0008-6223/$ - see front matter  2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.01.067

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reduction system consists of a KrF excimer laser (Lambda Physik Compex Pro 205), a quartz tube containing 15 mL of the GO solution and a magnetic stirrer. The picture of the experimental setup is shown in Fig. 1. The KrF excimer laser has a wavelength of 248 nm, a pulse width of 20 ns, maximum pulse energy of 700 mJ and maximum repetition rate of 50 Hz. The laser beam size is around 24 · 10 mm2. The laser beam is directed towards the quartz tube without using any focusing lens so as to obtain a large irradiation area (as shown in Fig. 1). The working distance between the laser beam port and the quartz tube is 30 cm. The quartz tube containing 15 mL of the GO solution was mounted and positioned over a magnetic stirrer. It was constantly stirred by a magnetic stirrer during the laser irradiation process. In a typical pulsed laser reduction experiment, the laser energy was 200 mJ, the laser repetition rate was 5 Hz, and the typical irradiation time was 5 min. The UV–visible absorption spectra were performed on a Cary 500 UV–visible–NIR spectrophotometer (Varian Co., USA). The microstructure was characterized by X-ray diffraction (XRD, Bruker D8 Focus) and Raman spectroscopy (JY super LabRam) using 632.8 nm wavelength laser. Elemental composition analysis was carried out using X-ray photoelectron spectroscopy (XPS, PHI 5000 Versa Probe). Thermo-gravimetric analysis (TGA) was conducted using a Shimadzu DTG60 instrument from room temperature to 650 C at a heating rate of 5 C/min under nitrogen. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet AVATAR 370 DTGS spectrometer with the KBr-technique. The surface morphology and nanostructure GO were observed by scanning electron microscopy (SEM, Hitachi S-4800) and Atomic force microscopy (AFM, Nanoscope IIIa instrument) operating in the tapping mode with standard silicon nitride tips. The resulting r-GO films were produced by vacuum filtration through a 0.22 lm mixed cellulose ester membrane and dried at 60 C in vacuum condition. The sheet resistance (Rs, kX/sq) of the r-GO films was measured by a four-point probe method using Hall Effect measurement equipment (Lakeshore).

Fig. 1 – Illustration of the experimental setup of pulsed laser reduction system. The inset is optical images of GO solution (15 mL, 0.1 mg/mL) before (a) and after (b) pulsed laser irradiation.

3.

Results and discussion

Fig. 1a and b show the dramatic color change of the GO solution with ammonia before and after the pulsed laser irradiation. Upon pulsed laser irradiation, the yellow–brown color instantaneously turned black, indicating that GO could be rapidly reduced by pulsed laser irradiation. The pulsed laser reduction of GO solution can be easily monitored by a timedependent UV–visible spectroscopy. Fig. 2 shows the UV–visible spectra of GO solutions as a function of laser irradiation time. As a comparison, the UV–visible spectra of the GO with ammonia solution and the as-prepared GO solution are also included in Fig. 2. Notably, the addition of ammonia inhibited r-GO aggregation after the pulsed laser irradiation (see the inset of Fig. 2). Considering the incomplete removal of the negatively charged oxide functional groups, the formation of aqueous r-GO dispersion can be attributed to a strengthened electrostatic stabilization under alkaline conditions with the addition of ammonia, as the repulsion between negatively charged r-GO should increase at higher pH values. [18]. Additionally, the GO with ammonia displays a little increase of the absorption in the whole spectral region compared to that of as-prepared GO. It is suggested that GO solution can be partially reduced by the introduction of ammonia. This result is consistent with the previous reports [18,19]. When the laser irradiation time was increased from 10 s to 5 min, the absorption was gradually increased in the whole spectral region, which suggests that electronic conjugation within the graphene sheets is restored while irradiating and reducing of GO [20]. However, it cannot be found the further increments in absorption after exceeding 5 min of laser irradiation, indicating completion of the reduction of GO within several minutes. The experiment is based on the observation that the pulsed laser reduction of GO solution is in a fast and controlled manner. XRD patterns of graphite powder, GO powder and pulsed laser irradiated GO powder are presented in Fig. 3. The XRD pattern of graphite shows a characteristic peak (0 0 2) of

Fig. 2 – UV–visible absorption spectra showing the change of GO aqueous dispersions (0.025 mg/mL) as a function of laser irradiation time. The inset is images of aqueous dispersions after the pulsed laser irradiation with (a) and without (b) the addition of ammonia.

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Fig. 3 – XRD patterns of (a) graphite, (b) GO and (c) pulsed laser irradiated GO powder. The inset is (b) GO and (c) pulsed laser irradiated GO powder.

graphite at 26.60. After oxidation, the (0 0 2) peak of graphite powder disappears and an additional peak at 11.38 is observed (shown in the inset of Fig. 3), which is corresponding to the (0 0 1) diffraction peak of GO. The d-spacing of GO increased to 0.777 nm from 0.334 nm of graphite powder, which is attributed to the oxide-induced O-containing functional groups [21]. These results suggest that the graphite powder has been completely oxidized. After pulsed laser irradiation, the diffraction peaks of GO disappear and the diffraction peaks of graphite are also absent, indicating that GO has been successfully reduced to graphene sheets but the r-GO sheets remain disorderly packed structure despite their strong p–p interaction. The changes of structure from GO to r-GO by the pulsed laser irradiation are also reflected in the Raman spectroscopy. The Raman spectra of GO and pulsed laser irradiated GO using 632.8 nm laser excitation are shown in Fig. 4. In the Raman spectrum of GO, the G band is at 1586 cm 1 corresponding to ordered sp2 bonded carbon and D band is at 1336 cm 1, which is ascribed to edge planes and disordered structures[22–24]. The Raman spectrum of the pulsed laser irradi-

Fig. 4 – Raman patterns of (a) GO and (b) pulsed laser irradiated GO powder.

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ated GO also exhibits the presence of G and D bands at 1584 and 1327 cm 1, respectively. It should be noted that the frequency of the G and D bands of the pulsed laser irradiated GO are very similar to that observed in the as-prepared GO. From the Raman spectra in Fig. 4, the D/G intensity ratio (ID/ IG) before and after pulsed laser irradiation is 1.03 and 1.08, respectively. The ID/IG is a measure of disorder degree and average size of the sp2 domains [21,23]. By using the empirical Tuinstra-Koenig relation [24], which relates the ID/IG ratio to the sp2 cluster size, it can be concluded that the sp2 cluster size is slightly reduced from 4.3 to 4.1 nm after 5 min of pulsed laser irradiation. To further illustrate the pulsed laser reduction from GO to r-GO, XPS was performed to characterize the removal of the oxygen groups. The XPS high-resolution C1s spectra of GO and pulsed laser irradiated GO are shown in Fig. 5. It is noted that the oxygen-containing functional groups (C–OH, C–O and C@O) have been significantly decreased after the pulsed laser irradiation. The C–O (epoxy and alkoxy) and C@O peaks of the pulsed laser irradiated GO are accounted for 8% and 7.6% in the C1s spectra area instead of 38.1% and 14.3%, in the case of the as-prepared GO. The results suggest that most of the oxygen-containing functional groups have been removed after the pulsed laser irradiation. As XPS analysis is highly sensitive only to surface composition, TGA was used to characterize the removal of bulk content of oxygen groups. Fig. 5c shows TGA heating curves of GO and pulsed laser reduced GO powder under N2 atmosphere with a heating rate of 5 C/min. Both GO and r-GO shows 13 wt% loss around 100 C due to water removal. However, the GO exhibits about 37 wt% loss around 220 C, which has been attributed to the loss of oxygen-containing groups [25]. In contrast, the r-GO shows 18 wt% loss at 220 C, which is much lower than that of the GO, indicating a decreased amount of oxygen-containing groups. Therefore, the removal of the thermally labile oxygen-containing groups by pulsed laser irradiation leads to increase the thermal stability of the r-GO. The reduction of oxygen-containing groups in GO by pulsed laser irradiation was also confirmed by FTIR spectroscopy (Fig. 5d). The spectrum of GO exhibits the presence of C@O (1726 cm 1), O–H (1402 cm 1), C@C (1623 cm 1), and C–O (1104 cm 1). After the GO was reduced by pulsed laser irradiation, all the characteristic absorption bands of oxygen-containing groups (O–H, C@O, and C–O) were nearly eliminated, indicating that such GO has been reduced to r-GO. Fig. 6 shows SEM photos of GO and pulsed laser irradiated GO. The thin folded sheet-structure of GO can be clearly observed as shown in Fig. 6a and b exhibits the r-GO sheets consist of randomly aggregated, crumpled sheets and forming a disordered solid, which is generated by pulsed laser irradiation. The absence of charging during SEM imaging indicates the pulsed laser irradiated GO sheets are electrically conductive. This qualitative conclusion was further confirmed by a four-point probe method using Hall Effect measurement equipment. The lowest Rs of the r-GO film we obtained is 53.8 kX/sq, this value is comparable to that of the bulk film produced directly from GO film to r-GO film by the irradiation of camera light, whose sheet resistance is 9.5 kX/sq [13]. Fig. 7 is AFM images of GO and pulsed laser irradiated GO dispersion in water after their deposition on a freshly cleaved

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Fig. 5 – XPS high resolution C1s spectra of (a) GO and (b) pulsed laser irradiated GO powder. (c) TGA curves of graphite, GO and r-GO. (d) FTIR spectra of GO and r-GO.

Fig. 6 – SEM photos of (a) GO and (b) pulsed laser irradiated GO dispersion in ethanol (0.05 mg/mL) after their deposition on a freshly cleaved alumina foil surface through drop-casting method. mica surface through a drop-casting method. It is clear that some r-GO sheets turn to randomly aggregated and others remain separated in the dispersion. The average thickness of rGO, measured from the height profile of the AFM image (Fig. 7b), is about 1.1 nm. Compared with the original GO sheets (Fig. 7a), both of them have almost the same thickness,

though most of the oxygen-containing groups were removed after the pulsed laser reduction. Pulsed laser has been often applied in the surface modification or thin film deposition. The essence of this technique is the creation of an effective heat zone by the laser beam which induces ablation of materials from a solid (or occasion-

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Fig. 7 – AFM images of (a) GO and (b) pulsed laser irradiated GO dispersion in water (0.05 mg/mL) after their deposition on a freshly cleaved mica surface through drop-casting method.

ally liquid) surface or localized modification surface [26]. In our circumstance, a pulsed KrF excimer laser with wavelength of 248 nm, frequency of 5 Hz, and laser energy of 200 mJ and beam size of 24 · 10 mm2 was used in this study. When the laser beam was incident on the GO flakes, the flakes absorbed the laser energy, and the energy was rapidly converted into heat. Although it is estimated that a substantial local temperature rise (>1000 C) accompanies irradiation by using a simple heat balance equating absorbed energy with temperature rise, the temperature of solvent is about 40 C during pulsed laser irradiation due to the existence of solvent. As the exothermic reduction of GO was reported at around 200–230 C, attributing to the decomposition of the oxygencontaining groups in the GO [27,28]. The pulsed laser irradiation should raise the temperature of the GO flakes above 200 C and result in the thermal deoxygenating reactions for reducing GO, which is similar to the photo-thermal reduction using a pulsed Xenon flash [13]. With stirring of the GO solution, the pulsed laser reduction can be continuously occurred. Therefore, the pulsed laser can rapidly deliver more than enough energy to remove the oxide groups for reducing GO.

4.

Conclusions

This work demonstrated a processing technique for reducing of GO solution by the pulsed laser irradiation. Combination of UV–visible spectroscopy, XRD, Raman, XPS, FTIR, TGA, SEM and AFM, we proved the majority of oxygen-containing func-

tional groups were removed by the irradiation of pulsed laser. Pulsed laser irradiation of GO exhibited an electrical conductivity. Compared to chemical and high temperature thermal methods, pulsed laser irradiation reduction is simplified, rapid, energy efficient and poisonous material free. Besides that, the pulsed laser reduction of GO in solution makes it particularly attractive for producing graphene composites by premixing with other materials.

Acknowledgments This research was supported by the National Natural Science Foundation of China (No. 50972091), and Prof. Lei Huang appreciated the support of The Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.

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