GRAPHENE OXIDE AND FUMED SILICA GRAPHENE OXIDE NANOCOMPOSITES MODIFICATION BY THERMAL TREATMENTS

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1 GRAPHENE OXIDE AND FUMED SILICA GRAPHENE OXIDE NANOCOMPOSITES MODIFICATION BY THERMAL TREATMENTS Simonpietro AGNELLO a, Aurora PIAZZA a, Antonino ALESSI a, Andrea MAIO b, Roberto SCAFFARO b, Gianpiero BUSCARINO a, Franco Mario GELARDI a, Roberto BOSCAINO a. a Dipartimento di Fisica e Chimica, University of Palermo, Via Archirafi 36, I-90123, Palermo, Italy. simonpietro.agnello@unipa.it b Dipartimento di Ingegneria Civile, Ambientale, Aerospaziale, dei Materiali, University of Palermo, Viale delle Abstract Scienze Ed.7, I-90128, Palermo, Italy. In the present study we investigate post synthesis thermal treatments up to 400 C of graphene oxide (GrO) prepared from commercial graphite and of GrO-silica nanocomposites prepared by a solution of commercial Fumed silica nanoparticles (average diameter 7 nm or 40 nm) and the GrO. The thermal treatments were carried out in air, vacuum or He atmosphere to highlight tunable changes. Two GrO batches with small differences in the D (~1350 cm -1 ) and G (~1580 cm -1 ) Raman bands have been employed to evaluate effects depending on preparation. Thermal effects have been monitored through the Raman spectroscopy focussing on D, G and 2D ( cm -1 ) bands spectral ranges. The experiments evidenced that the I(D)/I(G) amplitude ratio and the 2D region profile change during thermal treatments in the range below 180 C with maximum rate at ~120 C. At higher temperature, with maximum efficiency at about 200 C, only the D and G bands show modifications with a tendency to decrease I(D)/I(G) and reduce the G band width. The comparison among GrO batches and nanocomposite evidences that the thermal history during preparation and defective structures of GrO are key factors for the final material. Keywords: Graphene oxide, nanosilica, nanocomposites, thermal treatments, Raman spectroscopy. 1. INTRODUCTION The increasing interest in carbon based nanostructured materials like graphene, graphene oxide and carbon nanotubes is related to their potential applications in many fields of materials science, medicine and environment to cite some [1,2]. Among the above materials, graphene oxide (GrO) is of particular concern also because it is a starting product to prepare graphene, through for example chemical or thermal reduction, and shows interesting functionalization and tunable physical and chemical properties [3, 4]. Aside GrO, particular consideration has recently been demonstrated for GrO-silica composites because of the potentialities for electrical applications, their chemical inertia and stability toward ions exposure [5-7]. The main physical features of all of the above reported materials are typically investigated through different experimental techniques among which the Raman spectroscopy is particularly useful. In details, some of the properties of GrO and its by-products can be characterized through the presence of specific Raman bands, that are related to given features in the material structure [8-11]. The principal ones are usually named D, G, D, 2D (also historically known as G ) [8-11]. The D, G and 2D bands are typically the most intense and are peaked at ~1350 cm -1, ~1580 cm -1 and ~2680 cm -1, respectively [10-11]. The D band is less intense and can be observed at about 1620 cm -1 [10]. Finally, a combination mode D+D at ~2940 cm -1 has been observed together with other low intensity combination modes in the same spectral region of 2D [10,11]. A not univocal attribution to specific vibrational modes of these bands is present in literature, mainly due to the study of materials with not homogeneous preparation, since this latter is still research topic. Notwithstanding, the G band is almost universally attributed to vibration of sp 2 hybridized C rings, whereas the various D bands are

2 typically associated to defective structures of graphene arising from C sp 3 hybridization of plane or edges [10]. The relevance of Raman spectroscopy is particularly clear in concomitance with material changes induced during its thermal or chemical reduction. Indeed, the above reported bands show spectral changes as well as amplitude ratio change. This feature is significant due to the interest in graphene oxide modification for the tunability of electrical, optical and chemical properties by its reduction [11,12]. In details, a change of the I(D)/I(G) amplitude ratio has been observed during the GrO reduction [11-14]. Furthermore, the 2D region is composed of sub-band more sensitive to oxidation changes; this region features also relevant modifications associated to morphological properties of the graphene as surface roughness and number of layers [8, 10, 12, 13]. Due to the interest in GrO nanocomposite, the reduction properties of composite GrO-silica materials has also been recently considered in view of the potentialities of these systems [7, 15]. However, the overall reduction has been considered only for as synthesized materials, without post preparation processing, or for treatments at temperature typically above 400 C. In this context, in the here reported study we compare, mainly by Raman spectroscopy, the thermal treatment effects on GrO and composite GrO-silica nanoparticles in the low temperature range below 400 C showing that very similar effects are induced that are attributed to the properties of the employed GrO and preparation route. 2. EXPERIMENTAL The GrO was prepared by oxidizing natural graphite flakes (Gr) in presence of H2SO4/H3PO4 (9:1 volume) and KMnO4 [16] and then purified and exfoliated. Two different batches of GrO (named and, which differ each other for the surface content of alcoholic and carboxylic moieties) were prepared. The was further reacted with two samples of nanosilica to obtain GrO-silica nanoparticles composite. In details, commercial fumed silica Aerosil grades with specific surface 300 m 2 /g (named AE300, average particles diameter 7 nm) and 50 m 2 /g (named AEOX50, average particles diameter 40 nm), characterized by purity higher than 99.8 % by weight, were used [17, 18]. The was used to obtain nanocomposites with the AE300 and AEOX50. All the reactions were carried out in distilled water. The protocol involves three steps: (i) sonication, (ii) vigorous magnetic stirring at 120 C (heating plate temperature), (iii) hydrothermal method: the remaining solution was kept in magnetic stirring for 2 hours at 140 C (heating plate temperature) in order to promote the rapid evaporation of water. Furthermore, during these treatments a partial reduction of GrO to rgro occurred as observed by colour change. This procedure gives rise to hybrids that were not water-soluble and water resistant, named /AEOX50 and /AE300. XPS, FTIR, XRD, SEM, AFM analyses confirmed the oxidation/exfoliation of Gr into GrO [19]. Successive thermal treatments in air, in controlled atmosphere of He or in vacuum (~10-7 bar) have been done in the temperature range up to 400 C, by inserting the given sample in a muffle furnace previously heated at the treatment temperature. Only for the treatment in He a Parr reactor has been used, and the temperature ramp was opportunely arranged to keep the sample at the maximum temperature for 7h.Thermally induced changes have been monitored by Micro-Raman spectroscopy. A Bruker SENTERRA instrument with diode laser excitation at 532 nm and spectral resolution 9-15 cm -1, has been used. The laser power was fixed at mw to avoid sample modifications; furthermore, measurements in three different sample positions have been repeated for each treatment to check the homogeneity. Analysis of the spectra has been done after subtraction of a linear baseline in the range cm -1, for the D and G bands, and another linear baseline in the range cm -1, for the 2D region. The spectra have been successively normalized to the G band and to the central feature of the 2D region at ~2960 cm -1, respectively.

3 3. RESULTS The Raman spectra of the sample are reported in Fig.1. It is observed that the D and G bands, peaked at ~1360 cm -1 and ~1605 cm -1, can be detected (panel ), furthermore a composite 2D region is recorded. In details, this latter comprises a peak at ~ 2960 cm -1, with a broad shoulder on the low energy side, and another peak at ~3150 cm -1 (panel ). These features are compatible with those typically observed in GrO and the broad bands detected are remnant of the presence of defects [10,11]. The Raman signal from could be recorded only after a thermal treatment in air at 80 C for 24 h, before this a strong luminescence signal buried the Raman bands. The spectrum reported in Fig.1, shows that the has the same main features as the but with different amplitude ratios. Furthermore, the low energy shoulder in the 2D region is now more evident Fig. 1 Raman spectra of the, untreated, and, treated 24 h at 80 C. Panel, the D, G bands; panel, the 2D region. /AEOX50 /AE /AEOX50 /AE Fig. 2 Raman spectra of the sample and of the composites with silica nanoparticles: /AEOX50 and /AE300. Panel, the D, G bands; panel, the 2D region. The Raman spectra of the -silica nanocomposites are reported in Fig.2. These spectra show some modifications with respect to the pristine. In particular, the I(D)/I(G) increases for both nanoparticles composites, whereas the G band full width at half maximum (FWHM) decreases for the /AE300. As regards the 2D region, an overall change of the relative amplitude of the bands composing this spectral region is found. The nanocomposite preparation induces the increase of the low energy shoulder and a decrease of the high energy band at ~3150 cm -1, with differences depending on the nanoparticles used. To clarify the observed differences, a sample was repeatedly treated in air at temperatures between 80 C and 400 C for increasing time at each T until no changes in the Raman spectra were detected (data not reported). It was found that modifications in the Raman bands are induced for T<300 C and not for higher T. To further characterize the thermal effects, isochronal treatments have been carried out for 30 minutes at temperatures in the range 80 C-300 C. As shown in Fig.3, the changes occur gradually below 200 C for the 2D region, whereas the D and G bands feature a large change of the D band and minor changes in the G. Treatments at higher T do not change the 2D band, whereas the D band recovers its initial

4 amplitude but broadens somehow, and the G band shrinks. A treatment directly at 200 C but in He or in vacuum induces the same changes in the 2D region, and effects dependent on the atmosphere (He, vacuum or air) for the D, G bands with an I(D)/I(G) ratio 9 and 4, respectively. 200 C 180 C 160 C 140 C pristine pristine 140 C 160 C 180 C 200 C Fig. 3 Raman spectra of the sample thermally treated for 30 minutes at each of the temperatures reported in the legend. The arrows mark the spectra evolution as a function of the temperature increase. Panel the D, G bands; panel, the 2D region. Similar treatments have been carried out in and in the nanocomposites /AE300, /AEOX50. Band modifications similar to those reported for the have been found, but with peculiarities depending on the sample. To summarize these results, the spectroscopic features found as a function of the used T are reported in Fig.4. It is observed that can be analysed only after 100 C due to photoluminescence. The D band changed in and already below 180 C whereas, in the same T range, it is almost stable in nanocomposites; above this T an amplitude decrease is always found. A FWHM reduction of the G band is found in all the samples above 180 C. The 2D region bands feature very similar changes at all T in and with levelling effect at T>180 C; lower variations are present in nanocomposites. D band amplitude (arb. units) Raman signal at 2700 cm /AEOX50 /AE300 0 (c) /AEOX50 /AE300 G band FWHM (cm -1 ) Raman signal at 3150 cm /AEOX50 /AE (d) /AEOX50 /AE300 Fig. 4 Raman spectral features of all the samples as a function of the isochronal thermal treatment temperature. D band amplitude relative to the G band; G band full width at half maximum; (c) amplitude of the signal at ~2700 cm -1 and (d) at ~3150 cm -1 relative to the signal at ~2950 cm -1.

5 4. DISCUSSION The GrO and GrO-silica nanocomposites have been investigated by Raman spectroscopy after their preparation. The obtained materials have the typical spectral features of GrO derived materials, with Raman band amplitudes and profiles depending on the preparation. This aspect is evidence of the presence of an oxidation level and of defect content somehow dependent on the employed route. In details, small differences between and are found, but the nanocomposites spectra are more modified. Since the preparation of the composites requires some thermal steps, the observed changes could arise during these ones. The detailed study of spectral changes during thermal treatments at 200 C in controlled atmosphere of He, air or vacuum have shown that the I(D)/I(G) ratio depends on the peculiarities of the treatment as the time or the atmosphere. The series of isochronal thermal treatments in all the used samples have shown that the I(D)/I(G) ratio changes in and first increasing up to 180 C, and the decreasing up to levelling at ~260 C (see fig.4a). The nanocomposites show only the decrease at temperature larger than ~180 C, suggesting that their preparation route is equivalent to a thermal treatment series at lower temperature. This aspect is supported by the changes of the FWHM of the G band (see fig.4b) essentially occurring at t>160 C for all the samples. This kind of effect of the nanocomposites is present also in the modifications of the 2D region bands. In fact, the /AE300 sample has always the amplitude ratio 5 for the spectral band at ~2700 cm -1, and for that at ~3150 cm -1. These values coincide with the levelling ones observed in all the other samples after all the thermal treatments, and for this reason no changes could be explained for all the treatments in the /AE300. At variance, the /AEOX50 starts from the amplitude ratio, for the band at ~2700 cm -1, and, for the ~3150 cm -1, these out of level values imply that thermal changes could occur. Furthermore, the /AEOX50 modifications of the 2D region start almost at those temperatures where the amplitude ratio, for the band at ~2700 cm -1, and, for the ~3150 cm -1, are attained in the and, suggesting that the same modifications are occurring in all the materials. These aspects suggest that some of the processes induced by the thermal treatments in GrO occur during the nanocomposite preparation. Since the 2D band spectral region is linked to the presence of defective structures these results somehow demonstrate also the relevance of these latter in the production of nanocomposites. 5. CONCLUSION We have investigated different types of GrO and GrO-nanosilica composites by Raman spectroscopy and post preparation thermal treatments. Two temperature ranges have been found, one below 180 C where changes in the 2D spectral region are found in all the materials, and one at higher temperature range where only the I(D)/I(G) ratio changes. By the features of the temperature induced spectral changes of GrO different batches and of GrO-nanosilica it is concluded that the composites preparation involves structural defects of the GrO starting material. ACKNOWLEDGEMENTS The authors thank the people of the LAMP group ( for useful discussions. Partial financial support by the FAE-PO FESR SICILIA 2007/ project is acknowledged. REFERENCES [1] SCHAEFER, H.-E. Nanoscience. 1. ed. Springer, p

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