Article pubs.acs.org/Langmuir
Surface Area Measurement of Graphene Oxide in Aqueous Solutions Pedro Montes-Navajas,† Natalia G. Asenjo,†,‡ Ricardo Santamaría,‡ Rosa Menéndez,‡ Avelino Corma,† and Hermenegildo García*,† †
Instituto de Tecnología Química, CSIC-UPV, Universidad Politécnica de Valencia, av. de los Naranjos s/n, 46022 Valencia, Spain Departamento de Química de Materiales, Instituto Nacional del Carbón, INCAR-CSIC, Francisco Pintado Fe 26, 33011 Oviedo, Spain
‡
S Supporting Information *
ABSTRACT: Graphene oxide (GO) forms persistent dispersions in aqueous solutions up to concentrations of 0.2 mg mL−1. Addition of methylene blue (MB) to these aqueous dispersion of GO gives rise to the observation in optical spectroscopy of new absorption bands that are indicative of the formation of MB/GO conjugates. Four new absorption maxima have been characterized, and their intensity varies depending on the relative concentration of MB with respect to GO. Two of these bands appearing at 677 and 757 nm correspond to individual MB molecules adsorbed on neutral or acid sites of GO, respectively. Two other bands at 615 and 580 nm are attributable to adsorbed MB molecules showing interaction with other neighbor dye molecules at incomplete (615 nm) or complete (580 nm) surface coverage. Complete coverage of GO surface by MB causes the formation of a precipitate and the separation of the MB/GO conjugate. EDS mapping of carbon and sulfur atoms of MB/GO conjugate indicates the homogeneous distribution of MB molecules coating GO sheets. A simple and reliable protocol for surface area measurement and determination of the level of aggregation for GO dispersions in water has been proposed by determining the amount of MB that leads to the maximum intensity of the 580 nm band and precipitation of the MB/GO conjugate. Specific surface area as high as 736.6 m2 g−1 in the range of the theoretical value for GO has been experimentally measured for diluted GO solutions, but aggregation levels of 15% were estimated for GO concentration of 50 μg mL−1.
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INTRODUCTION The key feature of graphene (G)-based materials is a 2D surface of one-atom thick carbon atoms hexagonally arranged.1,2 Typically, G and related materials contain a distribution of platelets in which some single layer G is accompanied by few-layer or multilayer particles.3−5 Determination of the number of layers for G films deposited on flat substrates can be relatively straightforward by AFM, Raman, profilometry, or other image techniques.6−9 In contrast, many G applications are based on the use of these materials suspended in a liquid phase, and in this case, determination of the presence of single-layer G and the proportion of fewlayer platelets is considerably more complicate. Generally, characterization of G-based materials is carried out after drying the solution, by measuring certain properties of dry powders.10−14 It is possible that the degree of exfoliation and the surface area as well as other properties in suspension are different from those measured after drying, due to the unavoidable stacking and agglomeration upon solvent removal. In the present article, we will present a reliable procedure for determination of surface area of G oxide (GO) directly in aqueous solution based on adsorption of methylene blue (MB) dye. © XXXX American Chemical Society
Most of the articles in which MB has been used to determine the surface area of G-based materials directly in suspension are based on the method initially reported by Aksay and co-workers or slight modifications of their procotol.15 The procedure was applied for the measurement of reconstituted Gs (rGs) generated by thermal treatment of graphite oxide in which the strong evolution of gases generated in the reconstitution of G sheet produces exfoliation of graphite oxide leading to rG samples poorly dispersible in water.15 For this reason, the original surface area measurement with MB was carried out by suspending rG samples and an excess of MB in ethanol after presumably formation of MB/rG conjugate.15 Ethanol was removed and then water was added to dissolve selectively MB not associated with rG sheets.15 Although this method has been the most used for G surface area measurement in suspension, there are several issues that still remain unaddressed. Thus, it is of interest to determine what types of aggregates between MB and G materials are formed, what is the consequence of the formation of these aggregates with respect to the dispersibility Received: August 3, 2013 Revised: October 5, 2013
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dx.doi.org/10.1021/la4029904 | Langmuir XXXX, XXX, XXX−XXX
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Article
measurements this is the pH value at which all the visible absorption spectra were recorded. When an aqueous solution containing a fixed amount of GO (in the range from 8 to 850 μg mL−1) is used instead of distilled water and the same study of the influence of MB concentration on the visible optical spectrum is performed, distinctive features are observed. At low MB concentrations a new band at 679 nm is observed. This λmax is similar, although shifted with respect to the λmax of nonaggregated MB. We attribute this small (13 nm), but reproducible, shift in λmax to the association between MB and GO. As mentioned earlier, there is a recent precedent in the literature where this conjugate between MB and GO has been also observed, based on changes in optical spectroscopy as consequence of the overlap if π clouds of MB and GO.31 Further increase in the concentration of MB gives rise to the development of a new band at λmax 757 nm that is not observed in aqueous solution at pH 2.5 and we attributed to protonation of MB, based on the known behavior of this basic tricyclic dye in strong acids.34 As we have already commented, the pH of the solutions containing GO was relatively acidic. However, this value is insufficient to protonate MB and soluble inorganic acids at the same pH value do not lead to the observation of the 757 nm band.34 We propose that protonation of MB is driven by the strong association between MB and GO favoring proton transfer at the conjugate. The potential implications in catalysis of this acidity strength enhancement due to strong adsorption remain to be developed. It should be, however, commented that there have been claims of an apparent enhanced strength of acid sites on GO, and this strong acidity of GO has been applied to use these materials as acid catalysts for the roomtemperature acetylation of aldehydes36 and room-temperature epoxide ring aperture.35 Further increase in the concentration in the MB leads to saturation in the intensity of the 677 and 757 nm bands. In the latter case, we consider that all the acid sites of GO of sufficient acidity have been titrated by MB. In contrast to the saturation of the intensity of the 677 and 757 nm bands, a new band appearing at 615 nm starts to grow. This 615 nm band is close to the 611 nm band observed in pure water solutions for Haggregates for MB, and in analogy with solutions of MB in pure water it can be attributed to the interaction between MB molecules adsorbed on GO. The presence of this 615 nm band will be a consequence of the increase in the density of MB adsorbed on GO and the interaction between two adsorbed MB in close proximity. Further increase in the MB concentration leads to the appearance of a forth maxima at 580 nm. This band is not observed in pure water whatever the MB concentration, and we attributed this to the close packing between MB in the upper and lower faces of the GO sheets. In a certain way, this close packing would correspond to an aggregation state higher than dimers that apparently is not possible to achieve in aqueous solution in the absence of a support or scaffold to organize MB molecules in a closer packing. To illustrate these changes in the visible absorption spectra, Figure 1 presents selected spectra recorded for a fixed GO concentration upon increasing the amount of MB. It should be noted that the optical absorption spectra shown in Figure 1 have been corrected by subtracting the initial absorption of GO. GO absorption is presented in the Supporting Information and exhibits a continuous absorption in the complete visible range with a gradual increasing absorptivity toward shorter wavelengths, without any peak. Analogous spectra to those shown in Figure 1 have been also
of the sample, and how to perform measurements directly in aqueous solutions that is more convenient medium for hydrophilic G-based samples, such as the case of GO. With regard to the use of water as medium, adsorption of MB on GO in water has been reported16−30 and observed that GO can adsorb a large quantity of MB in the range of 220−240 mg g−1. This amount was, however, considered much lower than the maximum theoretical adsorption capacity that based on Langmuir adsorption models was estimated around 350 mg g−1.17 This strong interaction between MB and GO was not used to determine the surface area of the GO.17 On other precedents, based on absorption optical spectroscopy, it was observed that MB forms different species on GO depending on MB concentration characterized by different absorption maxima.31 However, also in these studies, no connection of dye adsorption with the possibility to determine surface area was established. In the present article, we will present a brief description of the changes in the visible absorption spectra of aqueous MB solutions in the presence of various amounts of GO that clearly support the formation of strong MB/GO conjugates that upon increasing GO surface coverage become insoluble and completely precipitate from the aqueous medium. We will also show how, by determining the maximum intensity of the absorption band associated with maximum GO surface coverage, confident estimation of the surface area of GO in aqueous solution can be obtained.
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RESULTS AND DISCUSSION We will start describing the changes in visible absorption spectroscopy of a solution containing a fixed amount of GO upon addition of increasing concentrations of MB. Then, we will assign each of these absorption bands to the assembly of MB to specific sites on GO. High-resolution TEM (HRTEM) of the MB/GO conjugates will be interpreted based on this assignment, and we will conclude showing how the observation of a specific band characteristic of maximum GO surface coverage by MB can be used to determine the specific surface area of GO solutions in water. Visible Absorption Spectra of Aqueous GO Solutions upon Addition of MB. MB is a basic tricyclic dye that, in aqueous solution, exhibits a characteristic absorption band at λmax 664 nm for diluted concentrations (
