Article pubs.acs.org/JPCC
Nonlinear Optical Properties and Broadband Optical Power Limiting Action of Graphene Oxide Colloids N. Liaros,†,‡ P. Aloukos,†,‡ A. Kolokithas-Ntoukas,§ A. Bakandritsos,§ T. Szabo,∥ R. Zboril,⊥ and S. Couris*,†,‡ †
Department of Physics, University of Patras, 26504 Patras, Greece Institute of Chemical Engineering Sciences (ICE-HT), Foundation for Research and Technology-Hellas (FORTH), 26504 Patras, Greece § Materials Science Department, University of Patras, 26504 Patras, Greece ∥ Department of Physical Chemistry and Materials Science, University of Szeged, Aradi vt. 1, H-6720 Szeged, Hungary ⊥ Department of Physical Chemistry, Regional Centre of Advanced Technologies and Materials, Palacky University, Olomouc 77146, Czech Republic ‡
ABSTRACT: In this work we report on the preparation of some aqueous graphene oxide (GO) dispersions and the investigation of their nonlinear optical response under visible (532 nm) and infrared (1064 nm), picosecond and nanosecond laser excitation. The GO colloids were prepared under specific and well-defined conditions resulting in finely dispersed heavily oxidized large GO sheets. In all cases, GO colloids were found to present large nonlinear absorption and negligible nonlinear refraction. The physical mechanisms responsible for their nonlinear optical response are discussed. In addition, the so-prepared GO dispersions were found to exhibit large broadband optical power limiting action for both pulse durations, comparable to that of C60 for visible laser pulses and much superior for infrared ones.
1. INTRODUCTION The versatile chemistry of carbon, functioning as a building block for many new stable and structurally fascinating carbon nanomaterials, has resulted in a huge resurgence of interest on studying carbonaceous matter at the nanoscale.1−4 Perhaps the most recent example is graphene5 and its derivatives6−8 and among them graphene oxide.9,10 While graphene has captured interest for its physical properties, graphene oxide (GO) is a key functionalized analogue.11 It facilitates much more efficient dispersion in solvents12,13 and thus much easier handling and applicability toward graphene-modified materials and devices,5 after its reduction. Particularly, GO’s dispersibility in water renders it an even more attractive system, due to health and environmental safety concerns and of course due to financial aspects emanating from the less required safety precautions (apart from high fire hazard risks14). In addition, the presence of hydroxyl and carboxyl functionalities significantly expands its chemistry15 for further derivatization.16 Among the various properties of graphene and its derivatives capturing the interest of researchers are their nonlinear optical (NLO) properties,16−18 arising from their extended π-conjugate system and the linear dispersion relation holding for their electronic band structure. In particular, GO is considered as an electronically hybrid material featuring conducting π-states (from the sp2 carbon sites) and σ-states (from its sp3-bonded carbons) exhibiting a large energy gap between them.15 The ability of tuning the ratio between the sp2 and sp3 fractions, e.g., © 2013 American Chemical Society
by means of reduction chemistry, can be a powerful tool for continuously tuning the band gap and therefore changing GO, in a controllable way, from an insulator to a semiconductor or to a graphene-like semimetal.19 Ideally, graphene is an atom-thick sheet of sp2-bonded carbon atoms arranged in a honeycomb lattice, while GO consists of a graphene sheet decorated with oxygen-containing functional groups such as hydroxyl, epoxide, and carboxyl groups. Among the consequences of the covalent bonding of oxygen to the carbon basal plane is the sp3 hybridization through σ-bonding. As a result strain is developed on the specific sites where oxygen bonding occurs, followed by the creation of some curvature. Therefore, by tuning the degree of oxidation of GO, being a two-dimensional network consisting of variable sp2 and sp3 concentrations, new possibilities are opened for tailoring the electrical, optical, and/or chemical properties of GO. In that view, the nonlinear optical properties of GO are expected to depend on the degree of oxidation of the graphitic sheet.20 However, the dependence of these properties on the degree of oxidation is not expected to be simple and straightforward because of the nonstoichiometric nature of GO. In fact, it is expected that these properties will be characterized Received: January 17, 2013 Revised: March 9, 2013 Published: March 11, 2013 6842
dx.doi.org/10.1021/jp400559q | J. Phys. Chem. C 2013, 117, 6842−6850
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quartz lens. The spot radii of the laser beams at the focus were measured using a CCD camera. The beam radii (i.e., half width at 1/e2 of the maximum of irradiance) of the 35 ps laser were found to be 17.5 and 30 μm, while those of the 4 ns laser were determined to be 18 and 31 μm at 532 and 1064 nm, respectively. The details of the Z-scan technique have been described in detail elsewhere.24,25 Briefly, from the “open-aperture” (OA) Z-scan recording, the nonlinear absorption coefficient β can be obtained by fitting with the following equation
by a complex interplay between the size, the shape, and the relative fraction of the sp2 and sp3 domains.21 In that view, the main motivation of the present work was the systematic investigation of the nonlinear optical properties and their correlation with the structure of some finely dispersed GO sheets in water. To shed more light onto the underlying physical mechanisms of the nonlinear optical response, experiments have been conducted using visible (532 nm) and infrared (1064 nm), picosecond and nanosecond laser excitation pulses, while special emphasis has been given to the preparation procedure and the characterization of the aqueous suspensions, the GO having been derived after specific treatment and under well-described conditions. In addition, the optical limiting action of these finely dispersed GO sheets in water at basic conditions has been studied, and it is discussed and compared with other existing reports. To the best of our knowledge, it is the first time that such GO suspensions have been studied for their nonlinear optical properties and optical limiting performance under visible and infrared, 35 ps and 4 ns laser excitation.
T=
+∞
1 ⎡ π ⎢⎣
βI0leff
⎤ ∫−∞
⎦ (1 + z 2 / z 02) ⎥
⎤ ⎡ βI0leff exp( −t 2)⎥dt × ln⎢1 + 2 2 (1 + z /z 0 ) ⎦ ⎣
(1)
where T is the normalized transmittance; I0 is the peak on-axis irradiance at the focus; z0 is the Rayleigh length; and leff is defined as: leff = [1 − exp(−α0L)]/α0 with α0 being the linear absorption coefficient and L the path length of the sample. The nonlinear absorption coefficient β is related with the imaginary part of the third-order nonlinear susceptibility χ(3) of the sample through the following relation
2. EXPERIMENTAL SECTION Synthetic and Preparative Procedures. GO was prepared according to the Brodie method22 as previously described.23 In particular, the current GO is the product derived after one time treatment with oxidizing agents. Aqueous suspensions were prepared by dissolving 2 mg of GO in 0.8 mL of H2O, which was previously calibrated at high pH with either NaOH, tetramethylammonium hydroxide, or ethylendiamine. Then the samples were sonicated (Branson 2510, 100 W, 45 kHz) for 20 min and centrifuged for 4 min at 1600g (5000 rpm, Heraeus Biofuge pico). The supernatant, a clear brown colloid, was isolated and further characterized. Transmission electron microscopy (TEM) images were recorded on a JEOL, JEM-2100 instrument operating at 200 kV, by evaporating a drop of a dilute aqueous suspension of GO. X-ray photoelectron spectra of GO were obtained by using an XR3E2 (VG Microtech) twin anode X-ray source and a Clam2 hemispherical electron energy analyzer. The used Mg K radiation (1253.6 eV) was nonmonochromatized. Survey scan spectra in the 1000−0 eV binding energy range were recorded with a pass energy of 50 eV. High-resolution spectra of the C 1s signals were recorded in 0.05 eV steps with a pass energy of 20 eV. After the linear baseline was subtracted, the curve fitting was performed by a mixed Gaussian−Lorentzian product function (30% Gaussian contribution). UV−vis spectra were recorded on a U-2800 Digilab Hitachi spectrophotometer. Thermogravimetric analysis was performed on a TA Instruments, Q500. An NTEGRA Aura (NT-MDT) microscope was used to monitor the quality of GO sheets. A drop of the diluted GO dispersion was placed on synthetic mica and evaporated at room temperature. The measurements were realized in air at ambient conditions in noncontact mode, with Si tips of the 1650−00 type at resonance frequencies ranging from 180 to 240 kHz. Nonlinear Optical Measurements. The nonlinear optical (NLO) response and the optical limiting (OL) performance of graphene oxide suspensions have been investigated by means of the Z-scan technique,24 using a 35 ps mode-locked Nd:YAG laser and a 4 ns Q-switched Nd:YAG laser employing both visible (532 nm) and infrared (1064 nm) laser pulses. In all cases, the laser beam was focused into the samples, which were contained in 1 mm thick quartz cells, using a 20 cm focal length
Im χ (3) (esu) =
10−7c 2n02 96π 2ω
β
(cm/W)
(2)
From the division of the “closed-aperture” (CA) Z-scan recording by the corresponding “open-aperture” one, and for small values of nonlinear absorption, the so-called “divided” Zscan is obtained, from which the nonlinear refractive index parameter γ′ of the sample can be deduced. γ′ is related to the real part of the third-order nonlinear susceptibility χ(3) through the following relation Re χ (3) (esu) =
10−6cn02 480π 2
γ′
(cm 2/W)
(3)
3. RESULTS AND DISCUSSION The study of NLO properties of GO suspensions in basic conditions is the difference in our approach as compared to previous studies.26−28 We have previously established,13 and later reported by other groups,12 that GO’s solubility is mainly attributed to the ionization of carboxylates and the electrostatic repulsions induced between the GO sheets. Very indicative of the aqueous behavior of GO is that prolonged stirring or sonication does not afford stable colloids, unless the pH of the medium lies in the region between 7 and 10.13 This observation, in turn, indicates that the deprotonation and ionization of carboxylic and hydroxyl groups is crucial for dispersing GO. Among various preparations tested herein, only NaOH and tetramethyl ammonium hydroxide resulted in fine aqueous GO suspensions, when the pH of the initial H2O was set between 11 and 11.5 for NaOH and between 11 and 12 for trimethyl ammonium bromide. Stable colloids were formed only when the pH of the starting H2O was set between the specified values because, probably, at lower pH values ionization does not take place extensively. On the other hand, at higher pHs (above 11.5−12) the ionic strength of the solution (due to base addition) increases considerably, leading to screening of the electrostatic repulsion between the charged groups of GO by the cations (Na+ and (CH3)4N+). Finally, 6843
dx.doi.org/10.1021/jp400559q | J. Phys. Chem. C 2013, 117, 6842−6850
The Journal of Physical Chemistry C
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Figure 1. TEM images showing GO sheets from the aqueous dispersion (a, b, c). AFM images and height profile of GO sheets from the same dispersion (d, e, f).
equivalent). Nevertheless, the obtained result, in our opinion, is a clear manifestation of the fine dispersion of the GO sheets and, roughly, of the dimensions of the colloidal entities therein. It should be also stressed that the distribution in Figure 2 skews to the right, up to a size of 1000 nm. This means that there are spherical equivalents detected with much higher size than 520 ± 100 nm. Despite the above explanations, there still appears to be a discrepancy between the TEM results (micrometer-range sheets) and the DLS results. It should be emphasized that the two techniques are complementary and, depending on the material studied, may provide different information. In TEM the sheets are completely dried, whereas in DLS the sheets are fully hydrated and in a dynamic state. In the present case, TEM clearly shows that GO sheets are full of wrinkles and foldings. Therefore, in the colloidal state, GO sheets are not expected to be extended and flat, but GO possibly (certainly from our point of view) forms smaller folded colloidal entities, which in turn may give rise to smaller equivalent hydrodynamic radii detected by DLS. In conclusion, DLS should mainly be considered here as a proof of the fine dispersibility of GO. In support of our findings, previously reported results29 also point to similar conclusions regarding the folded (crumpled) configuration of GO colloids. From zeta-potential measurements, a very high apparent surface charge (ζp = −67 ± 10 mV, Figure 2b) has been determined, being clear evidence of the highly negatively charge surface (i.e., presence of ionized carboxylate groups) as well as of the high colloidal stability of the sheets due to the corresponding electrostatic repulsions. The concentration of the obtained colloids was measured by drying water in a thermal analysis instrument with a weight accuracy of ±10 μg. Concentrations of the order of 1.2 mg/mL were attained in all cases, being further evidence of the high dispersibility of the present GO treated under the previously specified conditions. The GO from the stabilized colloid as well as parent GO were studied for their thermal stability under synthetic air atmosphere up to 700 °C. The respective thermograms are
ethylene diamine did not result in stable colloids at any pH value, possibly because of bridging between anionic groups from different GO sheets. In Figure 1, representative TEM and atomic force microscopy (AFM) images of the so-obtained GO suspensions are shown. As observable from these figures, large GO sheets of micrometer range are evident. It is quite interesting to note here that the present procedure for production of GO suspensions is quite effective since it is capable of producing stable colloids, despite the large size of the sheets. Moreover, preserving large GO sheets in suspension, rather than very small fragments, is attractive for their follow up utilization for the fabrication of films. In fact, free-standing films of GO were produced during water evaporation, when placed in the thermogravimetric analysis pans. In Figure 1e the height of a GO sheet is estimated to be about 1.8 nm. The thickness of airdried GO sheets has been reported to be in the range of 0.6− 0.7 nm.23 The dynamic light scattering (DLS) footprint of the GO colloids is shown in Figure 2a, showing mean hydrodynamic diameter of 520 ± 100 nm. This size should not be considered as an absolute size of the GO sheets. According to the principles of laser light scattering size measurements, this is the size of a spherical particle, having the same diffusion coefficient as the studied colloids (i.e., a diffusion-based spherical
Figure 2. (a) Dynamic light scattering size-distribution diagram of the GO suspension and (b) zeta potential distribution curve of the same dispersion. 6844
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Article
Figure 3. (a) Thermographs of the GO after drying from its fine suspension (suspended GO), as well as from the parent GO (untreated). (b) XPS spectrum of the parent GO.
shown in Figure 3a. It is deduced that the parent GO is thermally more stable than the suspended fraction. This observation should be ascribed to the fact that the suspended sheets are probably smaller than those of the parent GO. Furthermore, a higher density of functional groups (carboxylates and hydroxyls) in the final colloid is translated into a higher density of sp3 bonds, which culminates into a carbonaceous network more prone to oxidation under the conditions of the thermogravimetric analysis. The fact that the thermogram in synthetic air of the GO colloid does not reach zero (complete burning of the GO) should be attributed to the residue of the base (NaOH) which the colloid contained. The GO colloid was also tested for its thermal response under N2 flow. It shows a quite different thermal response (Figure 3a) in comparison to other GOs reported in the literature.30,31 More specifically, although the first mass loss of 30% is similarly observed at ca. 250 °C, the material is stable, unlike other GOs30,31 where after the end of the first decomposition step a continuous drop is recorded. The step-like decomposition of the present system probably indicates a material with welldefined structure, rather than a mixture of GO sheets of slightly different composition or sizes. Among the motivations of the present work was to investigate the effect of the degree of oxidation of GO on the NLO response. This issue is very interesting since, as it is known, the degree of oxidation is connected to the fractions of sp3 and sp2 bondings. For this reason we have used a highly oxidized GO, unlike GOs studied elsewhere,30,32 where the goal is the final formation of conductive graphene films. This is evident from the XPS spectrum of the parent GO, shown in Figure 3b. From this spectrum it becomes apparent that only a very small percentage (
