Letter pubs.acs.org/NanoLett
Hydration of Bilayered Graphene Oxide B. Rezania,† Nikolai Severin,*,† Alexandr V. Talyzin,*,‡ and Jürgen P. Rabe† †
Department of Physics and IRIS Adlershof, Humboldt-Universität zu Berlin, D-12489 Berlin, Germany Department of Physics, Umeå University, Umeå, SE-90187 Sweden
‡
S Supporting Information *
ABSTRACT: The hydration of graphene oxide (GO) membranes is the key to understand their remarkable selectivity in permeation of water molecules and humidity-dependent gas separation. We investigated the hydration of single GO layers as a function of humidity using scanning force microscopy, and we determined the single interlayer distance from the step height of a single GO layer on top of one or two GO layers. This interlayer distance grows gradually by approximately 1 Å upon a relative humidity (RH) increase in the range of 2 to ∼80%, and the immersion into liquid water increases the interlayer distance further by another 3 Å. The gradual expansion of the single interlayer distance is in good agreement with the averaged distance measured by X-ray diffraction on multilayered graphite oxides, which is commonly explained with an interstratification model. However, our experimental design excludes effects connected to interstratification. Instead we determine directly if insertion of water into GO occurs strictly by monolayers or the thickness of GO layers changes gradually. We find that hydration with up to 80% RH is a continuous process of incorporation of water molecules into single GO layers, while liquid water inserts as monolayers. The similarity of hydration for our bilayer and previously reported multilayered materials implies GO few and even bilayers to be suitable for selective water transport. KEYWORDS: Graphene oxide, graphene, hydration, scanning force microscopy
G
free conditions.6 However, there is no deeper microscopic insight into the hydration process, despite significant experimental and theoretical efforts.5−7,11−16 The difficulty originates not the least from the fact that GO is a rather complex material with a not well-defined structure, including several types of functional groups involved.6,17−19 With respect to the general similarity of GO hydration to the swelling of clay materials, the gradual changes of GO interlayer distances were assigned previously to effects of interstratification, that is, distinctly different and randomly stacked hydration states.5,7,9,10,20,21 The insertion of water into clay minerals occurs typically by a distinct number of monolayers with a thickness of 2.2 to 3 Å.22 The random stacking of the monolayers results in only one (001) reflection monotonously shifting upon hydration.8,9 Unlike in clays, distinct hydration
raphene oxide (GO) membranes have been shown recently to have remarkable permeation properties for gases,1,2 liquids, and vapors.3,4 For example, micrometer thick membranes have been demonstrated to be selectively and highly permeable to liquid water. This was attributed to the hydration of membranes, which is known to result in an increase of the GO interlayer distance,3,4 The degree of GO hydration is typically evaluated by diffraction.5,6 The d(001) reflection provides information about the distance between “graphene skeleton” planes (in the following named “interlayer distance”) but neither about functional groups attached nor about the true thickness of the pure GO layers. The hydration has been shown to occur in two steps: a gradual expansion of the interlayer distance at relative humidities (RH) below ∼80% and a steplike increase of the layer separation at higher RHs and in liquid water.5,7 The degree of hydration is also known to change gradually in polar solvents upon temperature and pressure variations.8−10 The water can be reversibly removed from the GO structure but complete drying is reported to be rather slow, even at humidity© 2014 American Chemical Society
Received: April 13, 2014 Revised: May 22, 2014 Published: June 12, 2014 3993
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states corresponding to the insertion of water monolayers have never been observed experimentally in graphite oxides and interstratification was never confirmed for GO membranes. On the other hand, the observed gradual expansion of every single interlayer in GO would be also a viable explanation for the experimentally observed monotonous shift of (001) reflection for GO membranes. Scanning force microscopy (SFM) imaging is a powerful method to evaluate interlayer distances of GO with only a few layers,23−25 but to the best of our knowledge such experiments were performed so far only at fix ambient (and often not specified) humidity. Here we report on SFM imaging of few layer graphene oxides, which we used to measure the interlayer distance (i.e., the step height between GO layers) as a function of humidity. This experimental design excludes effects connected to interstratification. Therefore, it is possible to verify directly and unambiguously if insertion of water into GO structure occurs strictly by monolayers or the thickness of GO layers can change gradually. Experimental Section. Precursor Brodie (C/O = 2.85) and Hummers (C/O = 2.47) graphite oxides used here have been characterized previously, including detailed studies of hydration and solvation properties of these materials over broad temperature and pressure intervals.8,10,12,13,26,27 Noteworthy, syntheses according to the Brodie28 and Hummers methods29 result in materials that are strikingly different with respect to many properties, including exfoliation and hydration/solvation.10,26,30,31 There is also a difference in hydration of GO powders and membranes.32 Dispersions of Hummers graphite oxide (HGO) in water were prepared using sonication for 12 h followed by centrifugation to remove few layer flakes. Brodie graphite oxide (BGO) was sonicated in an aqueous solution of NaOH (0.01 mol/L) as it could not be dispersed in pure water. Diluted dispersions were spin-casted onto a freshly cleaved mica surface (Ratan mica Exports, V1 (optical quality)). For this, a droplet of solution was applied onto a freshly cleaved mica surface for a few seconds and then spun off; this procedure was repeated three times. The scanning force microscope (Bruker Corporation, Multimode, Nanoscope 8) was operated with a J-scanner (unless specified differently) in tapping mode at a typical rate of 3 min per image. The SFM chamber was purged with either dry nitrogen to reduce the humidity or with nitrogen bubbling through a gas washing bottle filled with water (deionized and purified water, Protegra CS Systems CEDI Technology >10. MΩ·cm) to increase the humidity. Silicon cantilevers were used with typical resonance frequencies of 300 kHz and spring constants of 42 N/m. The tips exhibited a typical apex radius of 7 nm with an upper limit of 10 nm, as specified by the manufacturer (Olympus Corporation). SFM imaging in water (Aldrich, W4502) was performed with a fluid cell (Bruker Corporation) and with silicon cantilevers with typical resonance frequencies of 70 kHz and spring constants of 2 N/m. The SFM images were processed and analyzed with SPIP (Image Metrology A/S) image processing software. Relative humidities (RH) and temperatures were measured with a sensor (testo 635 of Testo GmbH) located in close proximity of the SFM head. The calibration fidelity of the sensor is ±2.5% RH in the addressed RH range, as provided by the manufacturing company. RH values indicated in the text are the displayed values. Results. Figure 1a shows a typical high-resolution SFM topography image of an HGO flake on mica. The demonstrated
Figure 1. (a) SFM height image of mica covered with single (I) and double (II) HGO layers. (b) Cross sections along the white dotted line in (a) corresponding to the fast scan direction. To compensate for thermal drifts and sample inclinations, the slope of single layer GO, that is, the gray-shaded parts, was fitted with a first order polynomial and the polynomial was subtracted from the whole line. This procedure was repeated for the whole image. (c) Histogram of (a) fitted with Gaussian functions (green and blue lines). The difference between mean values of the fits was assigned to the height of a GO layer.
image was processed in order to compensate influence of vertical drifts and sample inclinations. The image processing is illustrated in detail in Figure 1b, because it is important for the height analyses described in the following. The topography was fitted with a first order polynomial along the fast scan direction with one of the GO planes included and the other GO planes manually excluded from the fit. The height histograms (Figure 1c) were fitted with Gaussian functions and the difference in the mean values of the fits was assigned to be the height of the GO flakes. Figure 2a shows the height difference between single (I) and double (II) as well as between double (II) and triple (III) layer HGO flakes on a mica surface for one particular experiment. The height difference corresponds to interlayer distance evaluated using diffraction methods. The height of HGO flakes gradually grows with the relative humidity (Figure 2) from about 0.73 nm at 18% RH to more than 0.80 nm above 80% RH, but it does not return back to its initial value within 1 h of drying. Puzzled by the height hysteresis we repeated the experiment on a few different samples, both HGO and BGO, with qualitatively similar results (see Supporting Information), although the hysteresis for BGO appeared to be smaller as compared with HGO. 3994
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Figure 2. (a) HGO flake heights plotted versus time passed from the beginning of SFM chamber purging with wet nitrogen. The black squares and red circles show the height differences between single−double (I−II) and double−triple (II−III) HGO layers, respectively. At first, RH was increased from ambient 17 to 88% by purging the SFM chamber with wet nitrogen. Then starting from the time indicated with the dashed vertical line, RH was decreased to 1% by purging the SFM chamber with dry nitrogen. (b) The first part of the data from (a) for increase of humidity plotted as GO flake heights versus RH.
Figure 3. (a−c) SFM height images of the same area ontop of a double layer HGO flake imaged under (a) 3, (b) 5, and (c) 65% RH. The images have the same color scale of 4 Å from black to white. The images in (b,c) were manually squeezed and sheared to compensate the lateral thermal drift in order to align with (a). The white arrows indicate distinctive spots, that is, protrusions persisting from low to high humidities, used to align the images. The blue and yellow circles exemplify protrusions in (c), which do not exist in (a) and (b), and which are higher and lower than 3 Å, respectively. (d) Height histograms of (a−c) and the same area imaged under 60% RH (image not shown). The solid red and blue lines are the Gaussian fits to 5 and 65% RH histograms with the FWHMs of 1.3 and 1.8 Å, respectively. The histograms were normalized to have the same maximum.
image GO flakes only for a few minutes, until imaging became unstable, probably due to detaching of the GO flakes from the mica surface (see Supporting Information). Still, we could
While our experimental setup does not allow controlled measurements at humidities in excess of 90% RH, we performed imaging of GO directly in liquid water. We could 3995
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by ∼1 Å for both BGO and HGO (Figure 2 and Supporting Information). The interlayer distance then sharply increases by roughly 3 Å for both BGO and HGO in liquid water. The monotonic dependence of interlayer distance on RH is counterintuitive because the increase is smaller than the expected thickness of a water monolayer (about 2.2 to 2.9 Å).22 Yet, the values of interlayer distance we report here, measured as a function of humidity are in a good agreement with the humidity-dependent hydration experiments performed on very similar HGO and BGO graphite oxide powders.5,34 The change of interlayer distance (determined by X-ray diffraction (XRD)5) was gradual with a value change of about 1 Å in the humidity range 30−75%, followed by a sharp increase of ∼3 Å at 100% humidity. The gradual change was assigned to the effect of interstratification, that is, discrete hydration states (corresponding to insertion of certain number of water monolayers), randomly packed in multilayered samples, thus exhibiting a single peak in XRD patterns.5,7 The rise of RH in this model results in a growing number of interlayers intercalated with water monolayers, providing a gradual shift of the diffraction peak. However, interstratification can be excluded in our experiments with bilayered GO. The gradual expansion of bilayer GO implies that the simple model of two plates separated by water monolayers typically used in the literature is not valid for the description of GO hydration. Furthermore, the gradual increase of single GO flake heights implies that the process of intercalation of water molecules is a gradual process with water molecules incorporating continuously into variable sites as discussed below. Further increase of the interlayer distance obtained in liquid water occurs in a rather well-defined step of ∼3 Å, which can be attributed to the insertion of a water monolayer. The topographies of the GO flakes alter upon increasing the ambient humidity, exhibiting a granular morphology with protrusions and valleys with a typical apparent lateral extension in the range of 10 nm and height variations mostly within 3−4 Å (Figure 3). A substantial variation of the topography with varying humidity implies that the incorporation of water molecules contributes to the granular morphology. Comparison of the topography images made at low and high humidities (Figure 3) reveals three types of structures, which differ in their response to increased humidity: • Protrusions, which remain unchanged, can be attributed to an intrinsic roughness, possibly including water molecules within GO layers already at low humidity. • New protrusions with the height of roughly 3 Å can be attributed to water molecules intercalating between GO layers. • Protrusion with height variations smaller than 3 Å can be attributed to water molecules incorporating into the GO layer structure (see also Discussion below). The apparent width of a spherical object broadened by the SFM tip can be estimated as 2 Dd , with D and d being diameters of the SFM tip and the object, respectively. A molecule with a diameter of 3 Å would exhibit an apparent lateral size of roughly 4 nm for the SFM tips used here, that is, substantially exceeding the real size of the molecule. Therefore, it is difficult to discuss the number of molecules involved in the growth of protrusions. Yet, clusters of molecules must be smaller than the typical feature size, which is on the order of ten nanometers. A possible explanation could be that voids (or “valleys”) of variable size are present in the GO layer (including sizes smaller compared to the size of a water molecule). Incorporation of water molecules into voids with a size smaller
analyze the heights of the flakes imaged in water: 1.15 ± 0.03 and 1.25 ± 0.06 nm for HGO and BGO, respectively, exceeding substantially the heights measured in high RHs. SFM allows us also to follow changes of the surface topography during hydration. The surface of the HGO flake exhibits a roughness with typical apparent lateral patch sizes on the order of 10 nm (Figure 3). Comparison of high-resolution topography images of a HGO surface at low and high humidities indicates that the variation of the flake heights is accompanied by substantial topography alterations, where the surface roughness increases with humidity. The images had to be manually corrected for the thermal drifts in order to be compared. The corrections make it difficult to quantify topography alterations by subtraction of the heights, and particularly to follow the dynamics of the structure alteration. Yet, visual inspection of the images suggests three different types of structures (Figure 3). Some protrusions persist from low to high humidities, while other areas exhibit the growth of protrusions with variable heights with some protrusions increasing roughly 3 Å in height, as expected for the insertion of water molecules. Figure 3a shows a complicated topography pattern already for low humidity conditions, which reflects the well-known fact that GO sheets are oxidized inhomogeneously, exhibiting randomly distributed regions with different degrees of oxidation.33 Combining two GO sheets into a bilayered system provides a broad variety of possible water insertion sites: for example, in some areas both sides of the interlayers are highly oxidized and in others strongly oxidized areas can overlap with weakly or nonoxidized areas. It is expected that strongly oxidized areas are preferable for water insertion because they are more hydrophilic. Discussion. We attribute the observed variation of GO flake heights to the insertion/removal of water, which increases/ decreases the interlayer distance. Adsorption of water molecules onto the surface of flakes should not contribute to the detected variation of the thickness, because the height is measured from GO to GO surface. Furthermore, instrument calibration does not depend on the humidity in the addressed range (see Supporting Information). The SFM data imply rapid (on the time scale of image acquisition, that is, a few minutes) increase of the GO layer height upon RH increase. The maximum hydration of the graphite oxides is known to exhibit rather rapid kinetics (within 1−2 min) in liquid water.8 A similar hydration degree is known to be achieved also for multilayered samples at highly humid conditions (close to 100% RH) but with much slower kinetics (several hours for full saturation).5,6,21 However, it may be expected that bilayer graphene oxide with a small lateral size (a few hundred nanometers in our experiments compared to a few micrometers in bulk powders) can be hydrated faster compared to multilayered systems. The hysteresis in interlayer distance observed in our experiments with hydration/drying of graphene oxide can be explained by slower kinetics of water removal at low humidities. It is known that complete removal of water from GO powders takes weeks under drybox conditions,6 or alternatively requires annealing under vacuum at 70 °C for 24 h.33 The relatively weak interlayer distance hysteresis for BGO samples may be attributed to a higher degree and a more complex oxidation pattern of HGO, which results in more easy hydration of HGO as compared with BGO.26 The single interlayer distance increases monotonously with RH up to ∼80% with a gradual increase of interlayer distance 3996
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Notes
than the size of water molecules should expand the interlayer distance by the value that corresponds to the height excess of water molecule over the depth of the “valley”. Different sites for water adsorption in graphite oxides were proposed, based on the maximum in heat of adsorption observed in the range of a fraction of 0.2 to 0.4 of a monolayer.35 Our results support that water molecules adsorbed at humidities below 80% are inserted into specific sites of the GO structure, thus exhibiting no translational mobility as previously reported in neutron scattering studies.5,7,36 Note that regions with water inserted in protrusions were not migrating within roughly the 10 min required to capture two images. This allows us to record reproducible images from the same spot. Therefore, high-water permeation through membranes can be assigned to additional water layer inserted into the GO structure at very high humidities or in liquid water as suggested in previously published studies32,37 Insertion and removal of the water layer has been previously detected in temperature-dependent XRD studies of GO powder and membrane hydrations.9,32 Finally it is interesting to note that hydration experiments were performed here exclusively under nitrogen gas. It is known that dried GO membranes are vacuum tight,3,4 thus excluding the possibility of gas intercalation into the GO structure. However, recent experiments demonstrated nontrivial permeation properties of thin GO membranes for several gases under humid conditions.1,4 Therefore, it cannot be ruled out that GO hydration will be altered if experiments are performed using other gases. In summary, the change of the interlayer distance in bilayer GO was studied as a function of humidity and in liquid water, using SFM in order to measure GO−GO step heights. We found that the expansion of the GO interlayer distances occurs gradually at humidities below ∼80%. Hydration gives rise to a GO patchy surface with a typical apparent patch size below 10 nm and height variations in the range of 3 to 4 Å with no migration of the patches, that is, of the hydrated regions at a given humidity. This suggests that hydration through water vapor is a continuous process of incorporation of water molecules into various sites within the GO layers. Immersion of samples into liquid water increases the interlayer distance by another 3 Å (up to a total of ∼12 Å interlayer distance), suggesting the insertion of a water monolayer between the hydrated GO layers. Thus, the SFM imaging of GO−GO step heights allows to follow and understand the hydration of GO membranes and their molecular selectivity upon exposure to vapor mixtures, which can be used to separate binary gas mixtures with humidity modified 2−3 layer thick GO membranes.1 The similarity of hydration for our bilayer and previously reported multilayered materials3,8 implies GO few and even bilayers to be suitable for selective water transport.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Graduate School of Analytical Sciences Adlershof, SALSA, the Swedish Research Council, Grant 621-2012-3654, and by Ångpanneföreningens Forskningsstifelse (AT).
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ASSOCIATED CONTENT
S Supporting Information *
Verification of instrument calibration independence on relative humidity, data for all GO height humidity dependence experiments, and images of BGO and HGO in water with corresponding height histograms. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*E-mail: (A.V.T.)
[email protected]. *E-mail: (N.S.)
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NOTE ADDED AFTER ASAP PUBLICATION After this paper was published ASAP on June 17, 2014, minor corrections were made to the Figure 3 caption. The corrected version was reposted June 18, 2014.
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