Controlling the Thickness of Thermally Expanded Films of Graphene Oxide Xianjue Chen,† Wei Li,†,‡ Da Luo,† Ming Huang,†,⊥ Xiaozhong Wu,†,∥ Yuan Huang,† Sun Hwa Lee,† Xiong Chen,† and Rodney S. Ruoff*,†,§,⊥ †
Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea Department of Chemistry and ⊥School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea ∥ College of Chemical Engineering, China University of Petroleum, Qingdao 266580, People’s Republic of China
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ABSTRACT: “Paper-like” film material made from stacked and overlapping graphene oxide sheets can be exfoliated (expanded) through rapid heating, and this has until now been done with no control of the final geometry of the expanded graphene oxide material, i.e., the expansion has been physically unconstrained. (As a consequence of the heating and exfoliation, the graphene oxide is “reduced”, i.e., the graphene oxide platelets are deoxygenated to a degree.) We have used a confined space to constrain the expanding films to a controllable and uniform thickness. By changing the gap above the film, the final thickness of expanded films prepared from, e.g., a 10 μm-thick graphene oxide film, could be controlled to values such as 20, 30, 50, or 100 μm. When the expansion of the films was unconstrained, the final film was broken into pieces or had many cracks. In contrast, when the expansion was constrained, it never cracked or broke. Hot pressing the expanded reduced graphene oxide films at 1000 °C yielded a highly compact structure and promoted graphitization. Such thickness-controlled expansion of graphene oxide films up to tens or hundreds of times the original film thickness was used to emboss patterns on the films to produce areas with different thicknesses that remain connected “in plane”. In another set of experiments, we treated the original graphene oxide film with NaOH before its controlled expansion resulted in a different structure featuring uniformly distributed pores and interconnected layers as well as simultaneous activation of the carbon. KEYWORDS: reduced graphene oxide, graphene oxide paper, controlled expansion, thermal exfoliation, hot pressing
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New ways to control the dimensions and structures of such materials are clearly of interest. In this work, expanded rG-O paper-like material with a uniform and chosen thickness was achieved by spatial confinement during rapid thermal expansion. The thermal instability of graphite oxide (GO) was reported in 1859, when Brodie stated that crystals of oxidized graphite, “on the application of heat, are decomposed with ignition.”17 There is also an extensive body of early literature that discusses the thermal “deflagration” of GO.18−23 This process, now usually described as the “thermal shock” or “explosive” exfoliation of GO, is highly exothermic.24−30 This rapid thermal exfoliation can be also used for G-O films that have a similar chemical composition and interlayer spacing as do the layers of G-O in GO particles, with the G-O platelets in the G-O films randomly stacked/overlapped, whereas in GO the graphitic AB-stacking
raphene oxide (G-O) is a pseudo-two-dimensional material useful for constructing a wide variety of graphene-based structures. Directed-flow assembly of individual G-O platelets in a parallel fashion yields a paper-like material (“G-O paper”) with stacked, overlapped, and densely packed G-O platelets whose mechanical properties have been measured.1 Subsequently, there have been a large number of reports on chemically modified forms of G-O-based paper-like material, including films composed of chemically reduced G-O (rG-O),2 and films hosting other chemicals in the interlamellar spaces of the stacked/overlapping (r)G-O layers to achieve various interfacial interactions, e.g., hydrogen bonding,3 ionic,4,5 or covalent bonding,6 or π−π interactions.7 Other structural modifications of (r)G-O-based paper-like material have been made by directed assembly,8,9 chemical or thermal activation,10,11 laser writing,12 and controlled crumpling,13 among others. These modifications, covered in several review articles,14−16 have demonstrated structures and properties of (r)G-O-based paper-like material suggestive of a number of applications. © 2016 American Chemical Society
Received: October 15, 2016 Accepted: December 11, 2016 Published: December 20, 2016 665
DOI: 10.1021/acsnano.6b06954 ACS Nano 2017, 11, 665−674
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ACS Nano order is reportedly preserved.31 Depending on the initial C/O ratio of GO (or G-O paper or films), there is a 30%−60% mass loss in the thermally exfoliated product, due to the loss of H2O, and CO and CO2. The sudden heating of interlamellar H2O, and eventual loss of some CO and CO2, provides a “gas pressure” (reported as 1−2 orders of magnitude larger than the van der Waals forces (per unit area) holding the platelets together in GO),24 and this has been suggested as the main driving force for the rapid exfoliation. There are also reports of expansion under vacuum favoring the thermal exfoliation of GO at temperatures lower than 200 °C.32,33 It is possible to apply pressure to prevent significant expansion of the films upon rapid heating.34 Here we report a method of fabricating highly expanded (porous) and well-defined regions of rG-O films with controlled and uniform thickness by rapidly heating the G-O films with the expansion limited by a fixed gap above them. In short, the expansion is constrained by there being a “ceiling” beyond which further expansion cannot proceed. Figure 1a shows how G-O films were confined between two parallel quartz slides (25 mm × 25 mm × 1 mm) with a gap formed by placing spacers (pieces of Cu foils) of defined thickness (20, 30, 50, or 100 μm) between the slides. The slides were then clamped together in order to minimize the movement of all the components during the rapid heating. The monolayer G-O platelets, prepared by first preparing GO by the modified Hummers method35,36 and then dispersing it in water, roughly range from 100 nm to 10 μm wide, and the individual layers were measured to be about 1.2 nm thick by atomic force microscopy (AFM), Figure S1. G-O films (37 mm in diameter and ∼10 μm thick, Figure 1b) were prepared by vacuum “filtering” an aqueous dispersion of G-O through a cellulose nitrate membrane filter (47 mm in diameter with a 0.2 μm pore size); they were then separated from the filter. A photograph of the configuration is shown in Figure 1c, and in our experiments, this was rapidly heated by placing it on top of a hot plate preheated to 500 °C in air. A photograph of a piece of the thermally expanded rG-O film with a target thickness of 50 μm is shown in Figure 1d.
RESULTS AND DISCUSSION It has been noted in numerous reports that rapid heating of either GO particles (GO powder) or G-O films triggers a domino-like deoxygenating reaction that causes large volume expansion of ∼100−300× in a few seconds, accompanied by a change in color from dark brown for GO powder or G-O films to black for the exfoliated product material, rG-O.26 In the case of unconstrained thermal expansion of G-O films, the final film is typically broken into pieces or has many cracks, as shown in Figure S2. Scanning electron microscopy (SEM) images in Figure S3 show that rapidly heating a piece of an unconfined G-O film (∼10 μm in thickness) causes the film to expand roughly 100× in thickness, resulting in an exfoliated structure about 1 mm thick. However, the rapid expansion of spatially confined G-O films is restricted because the top and bottom surfaces of the expanding film cannot go farther than the confining quartz slides, and the film is thus limited in the height reached. The expansion under confinement is completed in less than half of a minute. There is an average weight loss of ∼51% for all the confined G-O films. In the process, the G-O films are thermally reduced to rG-O films, and evolved gases, including H2O, and CO and CO2 must escape in the horizontal direction. In contrast to unconstrained expansion, the film obtained here is never cracked or broken. A fracture surface of the original G-O film shows a ∼10 μmthick compact structure composed of stacked and overlapping G-O platelets, Figure 2a. The thickness of the expanded rG-O films, determined by the thickness of the spacer used, is roughly 20, 30, 50, and 100 μm, with parallel top and bottom surfaces, as shown in Figure 2b−e. The layers within the expanded films are loosely stacked in a roughly parallel fashion, which may be due to the way the evolved gas-phase molecules escape in the horizontal direction. For the “taller” expanded film regions, the layers become more undulating, and the average distance between the layers increases. The 20, 30, and 50 μm-thick expanded films have a relatively uniform structure across each of their cross sections, whereas the 100 μm-thick film is less uniform and contains larger voids. In all samples, the layers
Figure 1. (a) Schematic of the method for controlling the final height of rapid thermally expanded G-O films. (b) A photograph of a piece of separated G-O film prepared by vacuum filtering an aqueous dispersion of G-O platelets through a cellulose nitrate membrane filter and then peeling from the filter. (c) A photograph of the “confined expansion” configuration used. (d) A photograph of a piece of thermally expanded rG-O film with a target thickness of 50 μm. 666
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Figure 2. Cross-sectional SEM images of the (a) original G-O film (∼10 μm thick) and (b−e) thermally expanded rG-O films with thicknesses of 20, 30, 50, and 100 μm, respectively.
with much lower intensities than the peak for the G-O film, shift to higher angles that correspond to d-spacing values in the range of 0.36−0.42 nm. The decrease in the d-spacing is due to the elimination of interlamellar water and some oxygencontaining groups. The broadening of the fwhm suggests a decrease of the mean size of the ordered stack of the platelets.37 Besides, lattice distortions induced by the increase in the curvature and number of defects in the rG-O platelets could also contribute to the fwhm broadening.38 The presence of the main peaks at the higher angles indicates that the expanded films contain numerous domains of densely stacked/overlapping layers of rG-O platelets, consistent with the SEM and nitrogen sorption analysis results discussed above. The Raman spectra for all the samples show similar features, Figure 3b, where the intensity of the D peaks is comparable to that of the G peaks. Large D peaks with broad fwhms (95−100 cm−1) occur when the edges of G-O or rG-O platelets are sampled extensively along with interior regions where significant structural disorder and defects are also present.39 X-ray photoelectron spectroscopy (XPS) was used to analyze the surface chemical composition and bonding states of the G-O and rG-O films. C 1s XPS spectra for the original G-O film and the expanded rG-O films are shown in Figure 3c, with the full surveys given in Figure S7. The C/O ratio for the G-O films is 2.1, and the corresponding values for the expanded rG-O films are in the range of 4.7−5.9. The C 1s spectrum of the G-O film can be deconvoluted into three fitted peaks at binding energies of 284.6 eV for the sp2-hydridized carbon, 287.0 eV for the C−O (hydroxyl and epoxide) groups and 288.5 eV for the CO (carbonyl) groups;40 the C 1s spectra for all the expanded films are similar to each other, where, in comparison to that for the G-O film, the peaks for the sp2-hydridized carbon are sharper and have a larger intensity, and the peaks
appear to be cross-linked or adhering in places, and the pores are probably interconnected because they are channels for the released gas. Nitrogen sorption analysis shows low Brunauer−Emmett−Teller (BET)-specific surface areas of 17, 10, and 11 m2/g for the 20, 50, and 100 μm-thick expanded films, respectively, Figure S4 and Table S1, which implies that a large proportion of the layers consists of unexfoliated stacked and overlapping rG-O platelets. In comparison, the unconfined expanded rG-O film has a much higher BET-specific surface area of 349 m2/g. This result is consistent with the X-ray diffraction (XRD) pattern of the unconfined expanded film that shows a rather weak peak at 2θ of ∼24° for a significantly exfoliated layer structure, Figure S3d. We also performed the CO2 sorption analysis for the 50 μm-thick expanded rG-O film at 273 K, which can access micropores that are hardly (or not at all) detectable by the N2 sorption analysis at 77 K due to activated diffusion processes.29 The result shows a micropore surface area of 177 m2/g, Figure S5 and Table S2, indicating that most of the surface area of the expanded rG-O film is from micropores. We annealed a piece of the as-expanded 50 μm-thick film at 1000 °C in argon at atmospheric pressure, and the resulting film was almost unchanged in its geometry (Figure S6), although it was further deoxygenated (see Results and Discussion section below), indicating that it retains a “memory” of its earlier constraint. Figure 3a shows the XRD patterns of an original G-O film and the expanded rG-O films. A summary of the interlayer spacings and the full widths at half-maximum (fwhm) extracted from the XRD patterns is given in Table S3. The pattern of the original G-O film features a typical sharp and strong (001) peak at 2θ of 11.63°, corresponding to a d-spacing of 0.76 nm and a fwhm of 0.76°. The patterns for the expanded films appear to be similar (see details in Table S3), in which the main peaks, 667
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Figure 3. (a) XRD patterns, (b) Raman spectra, and (c) C 1s XPS spectra for the original G-O film and the expanded rG-O films with thicknesses of 20, 30, 50, and 100 μm.
bonds due to the cracking of epoxy groups.37 An XPS survey (Figure 4c) yields a calculated C/O ratio of about 37 for the 1000 °C pressed rG-O films. The C 1s spectrum in the inset has a sharp peak from the sp2-hydridized carbon as well as low intensity, broad peaks arising from C−O and CO groups. The (π−π*) shakeup satellite peak can also be seen. The C/O ratio (∼37) for the 1000 °C pressed rG-O film is much higher than that for the 1000 °C annealed rG-O films (∼15), which suggests that pressure along with heating promotes the reduction and “graphitization” of the rG-O films. The cross-sectional SEM images of the RT and 300 °C pressed films, Figure S9, show that the 20 μm expanded films are compressed to ∼10 μm and ∼7 μm in thickness, respectively. The 1000 °C pressed film, as the cross sections shown in Figure 4d,e, has a very uniform thickness of ∼5 μm, which is about half the thickness of the original G-O film (the d-spacing of the original G-O film (∼0.76 nm) is roughly 2.2 times of that of the 1000 °C pressed rG-O film (∼0.34 nm)). The ratio of the values of the interlayer spacing for the original G-O film and 1000 °C pressed rG-O films is consistent with the observation by SEM of the cross section of the “compacted” films after hot pressing at 1000 °C. Pressing at 1000 °C “compresses” the expanded film and also “graphitizes” it to yield a highly compact structure. (Here “graphitize” indicates the extensive formation of sp2-bonded “graphene-like” platelets that are stacked and overlapped and not the formation of an “AB-stacking” structure.) The sheet resistance of the 1000 °C compressed film is ∼5.9 Ω/sq. A report on the transformation of rG-O sheets to high-quality graphene by annealing at 1500 °C indicated that pressure helps
for C−O and CO groups are broader and have a smaller intensity. A low-intensity broad peak observed at a binding energy of 291.2 eV for all the expanded films can be assigned to a (π−π*) shakeup satellite peak that reflects the partial restoration of the aromatic carbon atom network.41 This result is consistent with our previous finding on rapidly expanded G-O films.34 The XPS spectrum (Figure S8) of the 1000 °C annealed 50 μm-thick expanded rG-O film has a C/O ratio of 15.1. Compressing Expanded rG-O Films. We were curious about what compression of such expanded films would do. As an example, 20 μm-thick expanded rG-O films were pressed at room temperature (RT), 300 °C, or 1000 °C. The RT and 300 °C pressings were performed on a Carver benchtop manual hydraulic press, and the pressing at 1000 °C was carried out in a hot press furnace (Jungmin Industry VSL). The XRD patterns acquired from the original, the expanded, and the pressed films are shown in Figure 4a. The peaks for the RT and 300 °C pressed films are barely changed compared to the as-expanded rG-O films, whereas that for the 1000 °C pressed film is shifted with a corresponding d-spacing of ∼0.34 nm (fwhm of ∼3.22°), which indicates that 1000 °C pressing further reduces the rG-O film and improves its graphitization. Raman spectra (Figure 4b) of the RT, 300 °C, and 1000 °C pressed films have been acquired. The spectra for the RT and 300 °C pressed films show similar features to that for the as-expanded rG-O film, whereas the spectrum for the 1000 °C pressed film shows sharper D and G peaks and an ID/IG ratio of 1.7. The high ID/IG ratio is possibly due to the large amount of disordered carbon generated by the destruction of aromatic carbon 668
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Figure 4. (a) XRD patterns of a 20 μm-thick thermally expanded rG-O film and pressed rG-O films made under different conditions. (b) Raman spectrum and (c) XPS spectrum of the rG-O film compressed at 1000 °C. (d, e) Cross-sectional SEM images of the 1000 °C compressed rG-O film.
promote the reconstruction of the sp2-bonded carbon network.42 There are also reports on the graphitization of turbostratic carbons or G-O at high pressure and at a temperature
