Article pubs.acs.org/JPCC
Tuning Radical Species in Graphene Oxide in Aqueous Solution by Photoirradiation Xue-Liang Hou,† Jing-Liang Li,† Simon C. Drew,‡ Bin Tang,† Lu Sun,*,† and Xun-Gai Wang*,† †
Institute of Frontier Materials, Deakin University, 75 Pigdons Road, Waurn Ponds, Victoria 3216 Australia Mental Health Research Institute, The University of Melbourne, Victoria 3010 Australia
‡
S Supporting Information *
ABSTRACT: Graphene oxide (GO) possesses unusual electronic and mechanical properties, including the ability to stabilize graphene radicals (GRs). However, controlled generation of GRs remains a challenge for applications requiring large-scale production. In this study, we demonstrate controlled production of GRs by UVB irradiation of GO solutions. Electron paramagnetic resonance spectroscopy of GO solutions revealed a dose-dependent exponential growth in radical production as a function of UVB exposure time. The GRs were air-stable over a long period, both in the solution state and in freeze-dried powders, suggesting they are graphene-based phenalenyl-like radicals. The redox activity of GRs was demonstrated by their ability to oxidize the chromophore 3,5,3′,5′-tetramethylbenzidine, with oxidation capacity of GO increasing with GR content.
1. INTRODUCTION Free radicals play a very important role in many fields such as catalysis, chemical synthesis, materials science, and biomedicine.1,2 Stable/persistent radicals are important for the development of materials such as electro-catalysts and spin probes/labels.3 However, to date, producing stable free radicals remains a challenge, due to the coupling of individual radicals. It has been demonstrated that graphite/graphene is a carrier of free radicals due to its large boundaries/edges and internal structural defects.4,5 The rigid π-conjugated planar structure of graphene makes it physically difficult for the radicals to react with each other, which means graphene may be able to serve as a carrier of stable free radicals. Recent efforts have focused upon theoretical prediction and computational modeling of GRs.6−8 However, in practice, their mass production has rarely been exploited. As an oxygen-rich graphite derivative, graphite oxide can be exfoliated into single carbonaceous layers, referred to as graphene oxide (GO), by mechanical agitation.9 GO is a general precursor for the production of graphene.10 Although the chemical structure remains ambiguous, it is generally accepted that GO is composed of a rigid and nearly flat carbon grid covalently linked with several types of oxygen-containing groups, such as carboxylic acid and phenolic hydroxyl.9,11 GO has an open surface and more edges compared with traditional graphite or graphene materials, which means it should be a superior carrier of graphene radicals. The presence of free radicals in GO has recently been observed.12,13 However, the origin of the radicals remains unclear. It has been proposed that they could originate from graphite or be produced during the © XXXX American Chemical Society
oxidation process due to the use of potassium permanganate.5,7,14 Moreover, the intrinsically low radical content of GO limits the practical applications of GRs. Therefore, it is very important to understand how to produce them in large quantities for practical applications and to obtain greater insight into their fundamental chemistry. It is known that heating or photo irradiation can produce free radicals.15,16 In this work, we observed that the content of GRs in GO can be easily tuned with photo irradiation, a well-known method for reducing GO.17,18 The GRs remain stable over a long period of time, which combines with the size tunability, good aqueous dispersibility, and excellent affinity of GO to various molecules, will contribute to the formation of a new class of stable free radicals with enormous opportunities.
2. EXPERIMENTAL SECTION 2.1. Materials. Graphite, HNO3, KMnO4, HCl (37%), H2O2, H2SO4, and 3,5,3′,5′-tetramethylbenzidine (TMB) were obtained from Sigma and used as received. Double distilled water was used in all experiments. 2.2. Preparation and Photoirradiation of GO Sheets. First, graphite oxide was prepared by a modified Hummers method.10,11,14 Second, GO was produced by ultrasonicating the graphite oxide in water for 30 min. The GO dispersion was then centrifuged at 7000 rpm for 30 min to remove the large sheets. The GO was then dialyzed against deionized water (4 Received: November 28, 2012 Revised: March 10, 2013
A
dx.doi.org/10.1021/jp311727t | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
L) for a week (with each water change every 24 h) to remove any impurities. Finally, the as-prepared GO solutions were placed directly under a UVB light source (AUV TL20−12, narrow band UVB, 100 W) for different durations. The energy of the light source (3.94−4.43 eV) exceeds the threshold of 3.2 eV required for photolysis. After UVB treatment, the samples were collected and adjusted to their original concentrations after irradiation. No aggregation was observed after irradiation. 2.3. Electron Paramagnetic Resonance (EPR). X-band continuous-wave electron paramagnetic resonance (EPR) spectra were obtained using a Bruker E500 spectrometer fitted with a Bruker superhigh-Q microwave cavity. Room temperature spectra were acquired using a Bruker AquaX cell (Bruker ER4110AX4). Frozen-solution spectra were obtained using a 5 mm OD quartz tube (Wilmad, 710-SQ-250M) inside a quartz coldfinger insert (Wilmad, WG-816-B-Q) at 77K. The magnetic field was calibrated using the g factor of the BDPA radical as a reference.19 Unless otherwise stated, the experimental parameters were set as follows: microwave frequency 9.867 GHz, microwave power, 10 mW; magnetic field modulation amplitude, 1 G; field modulation frequency, 100 kHz; receiver time constant, 164 ms; receiver gain, 85 dB; sweep rate, 1.0 G.s−1; number of averages, 50. Room temperature solution spectra were simulated, taking into account the experimental modulation amplitude and time constant parameters, using the WinSIM spectral simulation program.20 Frozen solution spectra were simulated using version 1.1.4 of the XSophe-Sophe-XeprView computer simulation software.21 2.4. X-ray Photoelectron Spectroscopy (XPS). XPS was used to characterize the surface functional groups of graphene oxide with and without UVB irradiation. A monochromatized X-ray source (He I, hν = 21.2 eV) was used. 2.5. Examination of Redox Activity of the Radicals. GO solutions (50 μg/mL) were mixed with TMB (1 × 10−4 M) and then incubated in the dark for various times. The absorption spectra of solutions containing TMB only, GO/ TMB mixture and GR/TMB mixture were then measured using a Varian Cary 300 UV−visible spectrometer. It was also used to characterize the light absorbance of GO without and with UVB irradiation for different durations.
Figure 1. EPR spectra of GO and UVB irradiated GO aqueous solutions. (a) EPR spectra of GO aqueous solutions without and with UVB irradiation (GO concentrations are 100 μg/mL) and (b) peak-topeak intensity of the EPR spectra in (a) as a function of UVB irradiation duration.
resuspended simply by shaking with hand. The good dispersible property should be due to the presence of the carboxylic acid groups that are not removed by UVB irradiation, which will be shown later from the XPS characterization. The g factor of paramagnetic molecular systems provides information on the electronic polarization and spin−orbit interactions of paramagnetic systems.6 The observed isotropic value of giso = hν/(βB0) = 2.0029 ± 0.0002 obtained from simulation (Figure S1 of the Supporting Information, SI) of the room temperature EPR spectrum (where B0 is the resonant field, h is Planck’s constant, ν is the microwave frequency, and β is the Bohr magneton) is consistent with that expected for organic radicals, which typically possess g factors close to the free electron value of ge = 2.0023. The observed value is also comparable with the g factor of localized radical signals observed for GRs. 12,22,23 The Voigtian line shape (a convolution of Lorentzian and a Gaussian) of the room temperature aqueous solution of GRs, with approximately 85% Lorentzian character and a narrow width of approximately 1.2 G (Figure S1(a,b) of the SI), suggests the presence of large intramolecular exchange interactions between unpaired spins in different regions of the GO plane. The EPR line shape did not significantly change as a function of UV irradiation time (although the signal-to-noise ratio at times
