The Effect of Thermal Reduction on the Water Vapor Permeation in Graphene Oxide Membranes

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The Effect of Thermal Reduction on the Water Vapor Permeation in Graphene Oxide Membranes

Konstantinos S. Andrikopoulos , Giannis Bounos ,* Dimitrios Tasis , * Lamprini Sygellou , Vassilios Drakopoulos , and Georgios A. Voyiatzis

hydrogen bonding interactions between adjacent GO sheets, whereas the spacing is controlled by the extent of intercala-tion of oxygen based molecular species. To this end, graphene oxide assemblies are considered as potential nanoporous membranes for selective separation studies. Permeability measurements of binary gaseous mixtures through ultrathin GO membranes (thickness less than 10 nm) has only been recently inves-tigated and the results showed enhanced selectivity towards the diffusion of spe-cifi c substances. [ 10,11 ] Concerning the permeation rates of neat vapor sub-stances, [ 9,12 ] it has been found that even small size monoatomic molecules, such as He, hardly diffuse through the pores

of thin GO membranes, whereas water vapor passes freely. [ 9 ] In order to explain the unimpeded water permeability through GO membranes (thickness from 0.1 to 10 µm), the authors suggested that a monolayer of water fi lls the network of gra-phene nanocapillaries and is capable of undergoing a low-friction fl ow in between the graphene channels. [ 9 ] Chemical reduction/deoxygenation of GO thin membrane at a specifi c temperature resulted in a decrease of water permeability by a factor of 100, due to the reduction in the channel width, which is too small for a water monolayer to form and fl ow. As com-plementary information to the experimental fi ndings, recent atomistic models for hybrid systems composed of water and graphene oxides [ 13 ] suggest that formation of hexagonal ice bilayer in between the fl akes is crucial to realize the perfect water permeation across the whole stacked structures. The distance between adjacent layers as well as the melting transi-tion of the ice at the fl ake edges which are assumed to be pas-sivated by hydrophilic edge groups was shown to determine the water fl ow rate. In contradiction, a recent work based on molecular dynamics (MD) simulations has shown that the fast slip fl ow for water inside the interlayer gallery between the graphene layers weakens with the presence of chemical func-tionalization and relaxation of nanoconfi nement in graphene oxides. [ 14 ]

It is obvious that the specifi c conditions needed for a tai-lored permeability of a gaseous substance through a GO mem-brane are far from clear and no predictions can be made on its behavior at different temperatures. In specifi c, the effect of both interlayer distance and chemical speciation of attached oxygen-containing groups to the permeation rate of water vapor needs to be clarifi ed in detail. For that reason, detailed studies on the

In the present study, the infl uence of the thermal reduction on the water vapor transmission properties of thin graphene oxide (GO) membranes is evaluated. The macroscopically measured property of the Water Vapor Trans-mission Rate (WVTR) exhibits step like dependence contrary to the gradual microscopic structural alterations identifi ed by several techniques (XPS, FTIR and XRD) applied in situ during the thermal annealing process. Three distinct regions of WVTR-values associated with distinct interlayer distances i.e., >7.5 Å, ∼6 Å and <6 Å are essentially observed which may be compared to the fi ndings of the recently reported fi rst principle calculations. Our experimental results enable the understanding of the water vapor unimpeded transmission through the layers of the oxygen rich GO nanostructured membranes and consequently facilitate the design of functional membranes for separation applications.

1. Introduction

Carbon-based materials have been extensively utilized either as wastewater purifi cation agents in the form of powder or barrier membranes. Especially in the recent decades, sev-eral works have appeared in the literature, concerning sepa-ration and/or permeability studies through active carbon, carbon nanotubes and graphene derivatives. [ 1–9 ] The scientifi c interest on GO, a material already known from the 19 th cen-tury, has re-emerged during the last years mainly due to its easy processibility towards the fabrication of multifunctional self-assembled structures. GO is actually an oxygenated gra-phene derivative with enhanced population of defect sites. By using simple processing methods, such as vacuum fi ltration, it is possible to prepare porous membranes consisting of well-packed GO platelets, analogous to the carbon nanotube-based buckypapers. The structural integrity of the formed macro-scopic sheets comes from a combination of van der Waals and

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Dr. K. S. Andrikopoulos, Dr. G. Bounos, Dr. D. Tasis, Dr. L. Sygellou, Dr. V. Drakopoulos, Dr. G. A. Voyiatzis Foundation for Research and Technology Hellas – Institute of Chemical Engineering Sciences (FORTH-ICEHT) P.O. Box 1414, 26504, Rio Patras , Greece E-mail: [email protected]; [email protected] Dr. D. Tasis Department of Chemistry University of Ioannina 45110 , Ioannina , Greece

DOI: 10.1002/admi.201400250

http://doi.wiley.com/10.1002/admi.201400250

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© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimwileyonlinelibrary.com1400250 (2 of 8)

time- and temperature-dependent thermal reduction of GO membranes [ 15 ] towards the fabrication of structures with tai-lored barrier properties are of great importance. In the present study, we experimentally examined the relationship between the various chemical/structural conditions of GO membranes and the water vapor transport rates. The reduction of d-spacing was achieved by using progressive thermal annealing at two dif-ferent temperatures (160 and 200 °C). The latter were selected due to the fact that gas substances, such as water vapor, CO and CO 2 , are released from GO samples by thermal disso-ciation of labile groups within this temperature window. [ 16 ] In addition, a quantitative and qualitative assessment of the GO interlayer distance as well as the population of the remaining oxygen-containing groups in the graphene basal plane and edges after thermal reduction was performed by means of X-ray diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS), and Attenuated Total Refl ectance Infrared spectroscopy (ATR/FTIR), respectively. The applied techniques give deep associ-ated insight about the role of both interlayer distance and popu-lation of hydrophilic groups on the water vapor transport rate.

2. Results and Discussion

2.1. Morphology of Cross Sections of Graphene-based Membranes

GO-based freestanding fi lms with a thickness ranging between 5 and 100 µm were obtained when an aqueous GO suspension was fi ltered through a polycarbonate membrane under vacuum conditions, followed by hot air drying. In order to assess the morphology of the resulting papers, cross sec-tions of graphene-based membranes were imaged by Scan-ning Electron Microscopy (SEM), before and after thermal reduction. In the representative images shown in Figure 1 A, the layered structure is obvious, due to the nature of deposi-tion process.

After thermal treatment, the reduction of the space between adjacent layers is clearly observed, as a result of elimination of oxygen-containing labile groups and trace amount of adsorbed humidity.

2.2. In situ XRD Analysis

The interlayer spacing of the starting GO material as well as of the thermally treated samples (at 160 and 200 °C), respec-tively, was estimated by in situ XRD measurements. In the XRD patterns of GO ( Figure 2 A), the strong and sharp peak at 2θ = 11.9° corresponds to an interlayer distance of ∼7.4 Å. [ 17 ] At the beginning of the thermal treatment of GO at 160 °C, a shift of the peak to 15.7° was observed, which corresponds to an interlayer distance of 6.13 Å.

Isothermal annealing at 160 °C for varying periods of time leads to the progressive shift of the XRD peak. Furthermore, after prolonged annealing, a second broad peak appears at 2θ∼25° which becomes more prominent with annealing time. This peak represents the structural changes of the GO structures due to the thermal elimination of oxygenated species originating from GO as well as of the adsorbed/intercalated water molecules. [ 18–20 ]

Similar observations, however experienced signifi cantly faster, are observed in the XRD patterns of GO membranes annealed at 200 °C (Figure 2 A). Isothermally annealed samples for a period of 1 hour at 200°C possess two peaks of similar intensities, the second of which corresponds approximately to that of conventional graphite (spacing 3.4 Å). [ 21 ] In situ XRD recordings on thermally treated GO samples were performed in a wide range of time intervals at the aforementioned tempera-tures. The intensity ratio of the thermally induced peak at about 25° (denoted as I G ) to the GO-originating peak (denoted as I GO ), as a function of treatment time is shown in Figure 2 B. For the case of annealing at 160 °C (solid circles), a change of the slope is observed after the fi rst 100 min. Similar structural changes are induced in GO lattice during the fi rst 10–15 min of the treatment at 200 °C (open circles). This clearly demonstrates that there is an appreciable difference in the rate of interlayer spacing decrease for the two studied temperatures.

2.3. In situ Spectroscopic Analysis of the Oxygen-containing Species

FTIR was employed for the structural characterization of the GO membranes at molecular level after thermal annealing.


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Figure 1. A. Sketch of the cup used for WVTR measurements and SEM images of the cross section of the GO membrane with and without thermal treatment at 200 °C for 1 h under vacuum. B. Homemade chamber for WVTR tests operating under controllable temperature and humidity conditions.

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Structural evolution of the membranes was studied as a func-tion of time for two different temperatures, namely 160 and 200 °C. Representative FTIR spectra at 200 °C for various treat-ment times are shown in Figure 3 A. The assignment of the most prominent vibrational bands is included in the fi gure and is based on the work of Acik et al. [ 22 ]

More specifi cally, the spectral region denoted as α involves vibrational bands of ethers (peroxides, furans, dioxolanes) as well as hydroxyls and carboxyls. Species such as perox-ides, pyrans and ketones contribute mainly in the β spectral region, whereas epoxides are the most common groups in γ spectral region with weaker contribution of peroxides, pyrans and ketones. Strong infrared absorbance bands appear in the

α spectral region for the multi-layered GO mainly due to C−O (all ether derivatives) and C-OH (all hydroxyls and species con-taining OH). The most prominent band of carboxyls is the one at ∼1720 cm −1 ; however only small contribution from ketones (C=O) is observed in the spectral range 1750–1850 cm −1 . The band at ∼1600 cm −1 is attributed to C=C of the graphene layers with some contribution from adsorbed/intercalated H 2 O bending modes. The most intense H 2 O bands appear in the spectral region ∼3000–3700 cm −1 along with the C-OH and car-boxyls of the GO structure (O-H stretching modes). Thermal annealing leads to the progressive alteration of the intensity of specifi c vibrational bands, the most characteristic of which are O-H modes and the modes appearing in the α and γ spec-

tral regions. Based on the fact that the con-centration of C=C bonds remains constant throughout the thermal treatment applied on the samples (see XPS analysis), we con-sidered the intensity of the corresponding vibrational band as a reference. Thus, the ratio of any other vibrational band intensity with respect to the reference one indicates the alteration of concentration of the respec-tive species. Figure 3 B depicts the depend-ence of carboxyls on the annealing time for both 160 °C and 200 °C as well as the cor-responding evolution of structural alterations associated with the species corresponding to the α and γ spectral regions. Figure 3 B also shows the decay of the concentration of the −OH species, belonging mostly to the adsorbed/intercalated water molecules, with annealing time. The concentration of carboxyls is approximately constant for both treatment temperatures, while the maxima of the bands in the α and γ regions along with the O-H stretching modes (assigned to both adsorbed/intercalated H 2 O and to related GO species) exhibit exponential-like reductions that strongly depend on annealing


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temperature. The decreasing experimental values reach pla-teaus at ∼120 min and ∼15 min for annealing temperatures at 160 and 200 °C, respectively.

In order to gain more insight concerning the effect of thermal reduction on the chemical structure of the GO mem-branes, XPS measurements were employed. A general observa-tion about the effect of thermal annealing is that the intensity ratios of C1s and O1s peaks change, due to the elimination of oxygen-containing species. As already shown in a recent work of our group, C1s and O1s spectra are analyzed into fi ve and three components, respectively, which were assigned to a wide variety of chemical functionalities. [ 23 ]

Concerning the C1s peak, the assignment is as follows: (1) sp 2 C=C bonding at 284.8 eV whose intensity is roughly constant for all annealing treatments, (2) sp 3 C-C bonding at 285.7 eV, (3) C-OH at 286.8 eV, (4) C=O at 287.8 eV and (5) COOH at 289.6 eV. The corresponding assignment of O1s peak is the following: (1) C=O at 531.1 eV, (2) C-O at 532.8 eV and (3) physically adsorbed H 2 O at 534.8 eV.

The evolution of the shape of C1s peaks after progressive isothermal treatment of GO at 160°C is shown in Figure 4 A. It is obvious that the components at higher binding energies, assigned to carbon-oxygen bonding, are suppressed appreciably after prolonged heating (5 h). Figure 4 B shows the time- and temperature-dependent evolution of some of the C1s and O1s components, which are indirect indicators of the relative popula-tion of oxygen-containing functionalities. These involve the total oxygen to total carbon (O/C) atomic ratio (star symbol) as well as the intensity ratios of C-O (rectangle symbol), C=O (circle symbol) and COOH (triangle symbol) to the total intensity of C1s peak. The latter ratios are calculated both at room tempera-ture and after thermal annealing, at 160 and 200 °C, respectively.

Before annealing, the total O/C ratio was 0.49. After the thermal treatment, the O/C atomic ratio decreases rapidly upon heating at 160 °C for 1 h and at 200 °C for 15 min and

continues to decrease with slower rates after prolonged heating at these temperatures reaching a value of 0.36 after heating for 5 hours at 160 °C or for 1 hour at 200 °C. From the intensity ratios of C-O, C=O and COOH components to the total C1s peak intensity, it was observed that after annealing at 160 °C for 1 hour, the C-O (hydroxyls and epoxides) decreased appreciably, whereas the C=O and COOH components did not change. The decrease in the amount of hydroxyl and epoxy groups vs. car-bonyl and carboxyl groups is also evident from the O1s spectra. Thus, the O/C atomic ratio decrease after heating in UHV is due to hydroxyl and epoxide moieties desorption. These data are strongly supported from the FTIR measurements, dis-cussed above.

2.4. Water Vapor Transmission Properties of GO-based Membranes

The alteration of the GO structure at molecular level is expected to cause changes in its macroscopic water vapor permeation properties. Representative curves of water losses as a function of time, for both non-reduced and thermally reduced GO mem-branes, using the wet cup method are shown in Figure 5 . Most of the tests lasted for 24 hours, however, few tests were extended to 48 hours (not shown) in order to verify the linearity trend of water weight loss over time. This linear correlation confi rmed that a steady-state water vapor transmission rate was achieved in the GO membranes. Minor deviations from linearity were observed only for the highly deoxygenated/reduced GO mem-branes and were attributed to the increased time required for the water molecules to intercalate within membranes in the humidity chamber. The WVTR values were calculated by the slopes of the water loss versus time plots in linear region and are given in Table 1 along with the interlayer distance extracted from ex-situ XRD analysis on the same samples.

The WTVR value for the non-reduced membranes is 2315 gd −1 m −2 , i.e., 2.5 times lower that the experimental value for the uncupped water vaporization which was measured to be 5750 gd −1 m −2 , under the same experimental conditions. This result is in a relatively good accordance with that of 0.5 µm-thick membrane reported previ-ously. [ 9 ] Thermal annealing at 160 °C for 1 h decreased the WVTR value by a factor of ∼1.7, compared to the reference value of non-reduced fi lm. This decrease originates from the structural alterations (primarily the removal of hydroxyl or epoxide chemical species) taking place in GO during thermal annealing, as confi rmed by the in situ FTIR and XPS results described in the previous section.

Figure 6 A,B depicts the correlation of WVTR values with O/C ratio and the GO quality obtained from in situ XRD experi-ments [expressed as the GO XRD peak con-tribution to the total XRD pattern i.e., I GO /(I G +I GO )]. In addition, Figure 6 C correlates



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Figure 4. A. XPS C1s core level spectra after (a) introduction in UHV and heating at 160 °C for (b) 1 h, (c) 2 h and (d) for 5 hours. B. Atomic ratio O/C (�), intensity ratio of C-O, C-OH component (�), O-C=O (Δ), and C=O (�) to all C1s peak intensity for prolonged heating at 160 °C (open red symbols) and at 200 °C (solid black symbols).

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the WVTR values with the d-spacing obtained from ex situ XRD experiments. It is interesting to note that, despite the fact that the structural modifi cations taking place in GO during its thermal annealing are progressive (see Figure 2 B, 3 B and 4 B), their correlation with WVTR appears as a step-like function.

From all the above dependencies, it is obvious that three separated reduction zones determine the water vapor permea-tion through GO-based membranes. The fi rst zone is extended for d > 7.5Å and corresponds to a mild reduction process. In this zone, the remaining H 2 O, hydroxyls and epoxides are responsible for keeping relatively large d-spacing. [ 20 ] Their rapid removal within the fi rst hour at 160 °C is probably critical on the evolution of WVTR value. The corresponding O/C ratio changes from 0.49 to ∼0.44 (Figure 6 A). The intermediate zone is extended in the range of O/C ratio values between 0.44 and 0.37 and the corresponding spacing is of about 6 Å, estimated by ex situ XRD (Figure 6 C). The main feature of that zone is the change of the mechanism responsible for water vapor transport which appears to be independent from the additional induced structural changes on GO. The third zone is charac-terized from negligible water vapor permeation. WVTR-value exhibits a drastic decrease for interlayer distances of about 5 Å. This result is in full accordance with the MD simulations [ 9 ] for

the determination of the bottleneck of the low friction transport of the water monolayer through capillary driven mechanism. The phenomenological contradiction between the progressive structural modifi cations of GO during its thermal annealing (especially in the transition from the mild to the inter-mediate reduction zone) and the step like behavior of the respective WVTR deserves some discussion.

Concerning the mild reduction zone, earlier in-situ studies upon the hydration behavior in GO multilayers [ 24,25 ] have shown that a continuous shift to higher interlayer distance is taking place with increasing the level of humidity. We examined the humidity-induced changes of d-spacing on two ther-mally treated GO samples. In the fi rst fi lm, reduction took place at 100 °C, whereas in

the second fi lm, at 200 °C. In the former case, the position of GO peak of the thermally treated GO fi lm at 2θ = 14.3° almost recovered to the initial state of the non-reduced GO (peak posi-tion 2θ = 12°) after 1 h exposure to a chamber with relative humidity (RH) 85%. By repeating the thermal treatment of the



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Figure 5. Water losses as a function of the time for the thermally treated GO membranes. A. at 200 °C for varying periods of time B. at 160 °C for varying periods of time as illustrated in the graphs. For comparison the results of non-reduced GO and open cup are illustrated as well.

Table 1. WVTR values derived from raw data of Figure 5 .A, B and inter-layer distances as determined from ex situ XRD analysis after thermal treatment.

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WVTR [gd −1 m −2 ]

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Figure 6. WVTR dependence on A. Oxygen to Carbon ratio (from in situ XPS), B. the percentage of GO contribution in XRD patterns (from in situ XRD) and C. d- spacing, for all the examined (reduced and non-reduced) GO membranes (from ex situ XRD). Solid circles represent the reduced GO membranes obtained after annealing at 160 °C and open circles at 200 °C at varying periods of time.

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previous fi lm at 100 °C for varying periods of time, a shift of the peak position to 2θ = 15° was observed. This behavior shows that the d-spacing experiences almost reversible alterations after successive cycles of water intercalation and desorption in between the thermally treated GO galleries ( Figure 7 A). In sharp contrast, the severely thermally treated GO membranes at 200 °C which possess a strong graphite-originating peak in their XRD patterns respond differently on their exposure to humidity. After an overnight exposure at 85% RH, a negligible change of the d-spacing (of 0.13 Å) was observed for the ther-mally treated fi lm (Figure 7 B). Because of the high sensitivity of the d-spacing to the humidity conditions, cautious investigation is required in order to study the mild reduction zone. Further experiments on water vapor permeability in the mild reduction zone are in progress.

Concerning the intermediate reduction zone, the most important fi nding is that, although the WVTR value remains constant, the sign from spectroscopic analysis indicates that structural changes continue to take place. In particular, for 1 h, 2 h and 3.5 h annealing time at 160 °C, the WVTR value was measured 1380, 1290, and 1370 gd −1 m −2 correspondingly. The latter data indicate a possible change of the water vapor trans-port mechanism, where the interactions of water molecules with the remaining oxygen species become signifi cant due to limited space for fl ow. Further annealing for 5 h seems not to fulfi ll the conditions for ultra-low friction fl ow [ 9 ] (d ≤ 6 Å), thus the WVTR value drops down to 320 gd −1 m −2 .

“Flash” time at 200°C resulted in reduction of the WVTR value to 1380 gd −1 m −2 . At these annealing conditions, the XRD pattern does not possess signifi cant graphitic features. Further thermal annealing for 15 min is associated with a WVTR value of 1110 gd −1 m −2 as well as the appearance of enhanced graphitic XRD peak at about 22°. Similar WVTR values were observed for the case of annealing time periods of 1–3.5 h at 160 °C. It is worth noting that, for all the above annealing conditions (i.e., 1–3.5 h at 160 °C and 0–15 min at 200 °C), the d-spacing of the ex situ experiments slightly varies between 5.83–6.09 Å. For the thermal annealing period of 0–15 min at 200 °C, in situ XRD analysis showed that the intensity ratio I G /I GO have reached to plateau (Figure 2 B). In addition, spectroscopic analysis at

such annealing periods showed that the total O/C ratio as well as the relative population of the majority of oxygen-containing groups, have reached their minimum value. Further increase of the treatment time to 1 h at 200 °C leads to the drop of WVTR value to 170 gd −1 m −2 , a value similar to that of the membrane annealed at 160 °C for 5 h.

In summary, considerable GO structural alterations take place in the intermediate reduction zone and are strongly interconnected with the water vapor permeation of the mem-branes. The population of hydroxylic as well as ether species decays gradually as a function of annealing time/temperature, whereas the graphite-like XRD component at about 25° appears in the respective data. However, the WVTR receives a specifi c value that is approximately half the value of the (reference) non-reduced GO membrane. On the contrary, in the extreme reduc-tion zone, almost all chemical species responsible for the water vapor permeation are eliminated, the graphitic-like peak in the XRD patterns is prominent and the WVTR value is diminished appreciably.

Despite the fact that in situ annealing measurements indi-cate progressive structural alterations of GO, the macroscopic property of WVTR exhibits a step-like dependence with the GO quality, which was assessed by XPS, FTIR and XRD. There are actually three distinct regions of WVTR-value associated with distinct interlayer distances i.e. d > 7.5 Å, d ∼6 Å and d < 6 Å. The fi rst mild reduction region is characterized by interlayer distances higher than 7.5 Å. This spacing value is in the range of the interlayer distances in stacked graphene oxide allowing fast water permeation (7−10 Å). [ 9,13,26 ]

Our experimental results may be explained within the concept of three models: the ultralow-friction capillary fl ow (UFCF) 9 , the weakened slip fl ow (WSF) 14 and the sliding of ice layers (SIL) 13 . The UFCF model was found to break down by molecular dynamics simulation whose results indicated that the presence of hydroxyl groups on the graphene sheets reduces the slip length which is also moderately further reduced after relaxation of the nanoconfi nement. The WSF model predicts that the slip length is increased with the reduction of hydroxyls in the GO structure; the increase is signifi cant for low hydroxyl concentration (<5%). Our experimental results indicate that the



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Figure 7. A. Alterations of d(002) peak position during the process of insertion and removal of water (annealing at 100 °C). B. XRD patterns of strongly reduced fi lm (200 °C) which has been exposed to 85% RH overnight at various temperature of retreatment. Arrow denotes the XRD peak shift.

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hydroxyl concentration that remain in the GO structure after prolonged thermal treatment is greatly diminished (Figure 3 B) while the permeability is reduced by at least one order of magni-tude. This is in contradiction to the predictions of WSF model, which clearly states that the side-pinning effects from the reduced GO walls is severely weakened, leading to high values of slip length, in the absence of hydroxyls in the GO structure. A modifi cation of the WSF model [ 27 ] attempts to understand the water permeation in GO through layer-by-layer as well as porous microstructures whose control could enable the design of functional membranes for various applications. The modi-fi ed WSF model may explain the decrease of the WVTR in the reduced GO membranes if only the pore size and/or density were altered accordingly; nevertheless, it is diffi cult to explain the step-like behavior in Figure 6 C.

It is interesting to check the behavior of the SIL model where ordered ice bi-layers in between the GO galleries as well as melting transition of ice at the edges of the fl akes explain the water permeation across a GO membrane. Although it is diffi cult to realize the existence of ordered water within GO galleries due to the high O/C ratio in our samples, the latter cannot be totally excluded because of the fact that the O/C ratio does not reach low levels after severe thermal treatment. The preservation of several structures such as carboxyls after the thermal treatment may indicate that a great number of defects in our samples exist at the edges of the fl akes. Since our data cannot indicate a specifi c structure for the water within GO gal-leries this issue remains an open question requiring further investigation. Nevertheless, the step-like behavior of WVTR may be explained by SIL model.The authors in Ref. [13] con-clude that the distance between GO layers, which can be con-trolled by oxygen reduction process or pressure determine the water fl ow due to the water dynamics in randomly connected two-dimensional spaces. They claim that the formation of water bilayer in GO membranes is crucial for the perfect permeation of water. The number of water layers formed within two succes-sive GO fl akes may qualitatively explain the step like behavior of the WVTR values in our experimental data. Thus, in agree-ment with the optimal interlayer distance range d = 7–10 Å for allowing fl owing of ordered water bilayer [28] as described in Ref. [13], the maximum experimental WVTR values are measured for d≈7.5 Å or else in the mild reduction zone of GO. In the intermediate reduction zone the reduction of d to ≈6 Å, which is close to the minimum interlayer distance (5.74−5.87 Å) [ 25 ] for intercalated water in graphene oxide, causes ∼50% reduc-tion of the maximum WVTR value. This may be attributed to the formation of water monolayers within GO galleries. Finally, excessive thermal treatment leads to a minimum value of the WVTR, which is attributed to the decrease of the d values below the minimum interlayer distance for single water layer interca-lation i.e. ≈5 Å. In full agreement with the SIL model are the experimental observations related to COOH groups. According to FTIR analysis, these groups remain un-dissociated in all examined annealing conditions extended from the mild to strong reduction zone. The presence of COOH groups at the edges of graphene oxide fl akes stimulates the destruction of ice bilayer with simultaneous penetration of water molecules to the next layer where they form another ice bilayer. Thus, the former result indicates that the processes of migration of the

ice layer in the plane or a shift of water molecules from top layer to the bottom one with further destruction (melting) of ice are possible to contribute in the complete picture of water permeation mechanisms.

Modeling of complicated structures such as GO is particu-larly diffi cult. The diffi culty is furthermore increased due to the inherent diffi culties posed by the intricate physical phenomena that dictate the transport properties of severely confi ned water molecules. On the other hand experimental processes introduce a number of parameters that are diffi cult to control in order to realize reproducible and reliable results. For example, two extreme experimental processes one referring to the prepara-tion of fi lms after controlled fi ltration of GO and another refer-ring to preparation of a pellet after excessive pressure on the same batch of pristine GO powder lead to materials possessing practically oppose transport properties. Things are even more complicated since GO with various structural characteristics may be prepared after following different preparation methods. From the above short discussion and taking into account that the physical phenomena involved in each case may differ (fl ow dynamics vs diffusion) it is apparent that any direct compar-ison of the experimental results with those from simulations is rather precarious. Thus, comprehensive experimental work as well as modeling investigations are both required for the under-standing of the basic concepts lying behind the macroscopic transport properties of GO and the ways to control it.

3. Conclusions

The WVTR of GO membranes has been studied as a function of annealing time and temperature. In parallel we have employed in situ and ex situ structural characterization of the annealed membranes by a number of complementary techniques (XRD, XPS, FTIR). The structural alterations involve mainly hydroxyls and etheric groups while the carboxyls are much less affected. The WVTR exhibit a scalable behaviour within three distinct zones: (a) a mild one where although considerable structural alterations are taking place, the WVTR exhibits high values, (b) an intermediate one where the structural modifi cations are more extensive and the WVTR is reduced to approximately half its maximum value and (c) an intensive (strong) one where the WVTR receives minimum values which are less than one order of magnitude compared to the maximum value. The phenome-nological contradiction between the progressive structural mod-ifi cations of GO during its thermal annealing and the step like behaviour of the respective WVTR is based on the adsorbed/intercalated water that fi lls the interlayer spacing and is dis-cussed within the framework of recently published models.

Experimental Section Material Preparation: GO was prepared by adopting a modifi ed

protocol from the work of Kovtyukhova et al. [ 29 ] It consists of two steps as recently described. [ 23 ] For the preparation of µm-thick membranes, a certain quantity of GO was exfoliated in aqueous medium by bath sonication for a period of one hour. Aggregated particles were discarded through centrifugation of sonicated GO suspension at 4000 rpm for 20 min. Thus, individually suspended GO platelets were remained in the



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© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimwileyonlinelibrary.com1400250 (8 of 8) Adv. Mater. Interfaces 2014, 1400250


supernatant part at a concentration of about 1 mg/mL. The latter was fi ltered through a polycarbonate membrane (pore size 400 nm) under vacuum conditions, dried by hot air and peeled-off from the membrane fi lter. The resulting membrane (GO buckypaper) has an average diameter of 7 cm and thickness ranging from 5 to 100 microns, depending on the starting quantity (initial loading).

Reduction/deoxygenation of GO membrane was carried out by a thermal process. The GO fi lm was heated up to 160 °C and remained in that temperature for 1 h, 2 h, 3.5 h and 5 h, respectively. In a parallel experimental run, samples were heated up to 200 °C and remained for 0 min, 15 min and 1 h, respectively. The heating was done under vacuum conditions. This thermal process resulted in the preparation of reduced/deoxygenated GO.

Characterization Techniques: A Specac heated golden gate controller – sampling accessory was used to measure the in situ temperature- and time-dependent Attenuated Total Refl ectance (ATR) spectra. The spectra were recorded by using a Bruker Equinox FTIR system, with spectral resolution 4 cm −1 . Each spectrum results from the averaging of over 30 scans.

The surface analysis studies were performed in a UHV chamber (P < 10 −9 mbar) equipped with a SPECS LHS-10 hemispherical electron analyzer. The XPS measurements were carried out at room temperature using unmonochromatized AlKa radiation under conditions optimized for maximum signal (constant ΔΕ mode with pass energy of 36 eV giving a full width at half maximum (FWHM) of 0.9 eV for the Au 4f7/2 peak). The analyzed area was an ellipsoid with dimensions 2.5 × 4.5 mm 2 . The XPS core level spectra were analyzed using a fi tting routine, which allows the decomposition of each spectrum into individual mixed Gaussian-Lorentzian components after a Shirley background subtraction. The GO samples were mounted on a stainless-steel holder heating plate able to reach ∼1000 K where the samples were heated at two temperatures 160 °C and 200 °C in situ, prior to XPS measurements. XRD measurements were performed in a Bruker AXS system (type: D8 Advance), equipped with a furnace.

The wet cup method, described by ASTM E 96–95, [ 30 ] was used to measure water vapor transmission rate (WVTR) through the GO and RGO membranes. WVTR is determined in the steady state water vapor transport conditions as water vapor fl ow rate through unit area of a body, normal to specifi c parallel surfaces, under specifi c conditions of temperature and humidity at each surface. According to this method, the test membrane covered a Petri dish fi lled with distilled water, as shown schematically in Figure 1 . The mass of water lost from the cup was monitored as a function of time, and the WVTR was calculated from the steady-state region. The examined membrane area was A = 10 cm 2 . The system cup-water-membrane was weighed and placed in a homemade chamber with controllable conditions of temperature and humidity (37 °C, 50% RH). The chamber consists of a home-made cartridge heater for temperature control, two N 2 inlets for controlling humidity (one for dry N 2 and another one for N 2 passed through water) and an axial fun for air circulation (Figure 1 B). To avoid any disturbing activity, a weighting balance interfaced with a computer is enclosed in chamber to record the weight as a function of the time. The slope of the water loss as a function of time normalized to the testing area A was taken as:

= ×

2WVTRMass H O lost

time area (1)

with units of gd −1 m −2 .

Acknowledgements The present work was partially funded by Synergasia 2009 program (project code: 09SYN-1156). This program is co-funded by the European Regional Development Fund and national resources.

Received: May 21, 2014 Revised: July 3, 2014

Published online:

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