Article pubs.acs.org/JPCC
Spectroscopic Study of Graphene Oxide Membranes Exposed to Ultraviolet Light Birgit Schwenzer,† Tiffany C. Kaspar,† Yongsoon Shin,† and David W. Gotthold*,‡ †
Physical and Computational Sciences Directorate and ‡Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States S Supporting Information *
ABSTRACT: Hierarchically stacked graphene oxide (GO) membranes have been observed to undergo physical and chemical changes under ambient conditions. In this study, the slow degradation of GO membranes is mimicked and accelerated by UV light (254 nm) exposure. Spectroscopic and X-ray diffraction analyses confirm that the observed changes, including macroscopic as well as atomic-scale alterations, cannot be attributed to loss of intercalated water molecules between the GO sheets, but UV light exposure triggers a partial reduction of the GO membrane to a more graphene-like material.
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INTRODUCTION Research on graphene oxide (GO) as anything but a precursor material for synthesizing graphene started to pick up in 20061,2 and was soon followed by a first report of freestanding GO membranes (also referred to as GO paper) from R. S. Ruoff’s group at Northwestern University.3 The first GO membranes were prepared by vacuum filtration. More recently, larger scale GO membranes have been prepared by tape casting4 and other methods.5 In step with the development of new fabrication techniques, GO membranes are now tested for a wide array of applications6 ranging from energy-related4,7 and biomedical8 applications to more conventional uses for filtration9 and dehumidification.10 For all these proposed and implemented applications it remains to be seen how sensitive each of them is with respect to chemical and physical changes of the GO membranes over time. In this study, we report the effects of UV exposure on 2Dhierarchically stacked GO membranes. Macroscopically observable changes, such as darkening and mechanical deformation, have been correlated to chemical changes at the molecular level through spectroscopic measurements. Not only do the results of this work offer insights into the stability of GO membranes under UV light, but the findings will enable researchers, who are studying the use of these materials for different applications, to better understand the shelf life and packaging requirements for GO membranes. Furthermore, our results demonstrate the feasibility of deep ultraviolet (DUV) photolithography for graphene oxide-based devices. This approach is readily scalable as opposed to previous reports on photolithographically patterned reduction of GO to graphene by AFM,11 electron beam,12 or with an extreme ultraviolet (λ = 46.9 nm) laser.13 © 2016 American Chemical Society
EXPERIMENTAL SECTION Synthesis. Graphene Oxide (GO) Membrane Synthesis and Fabrication. Graphite powder (Asbury, catalog # 3763; size: 500 μm) was oxidized using the modified improved method reported by Marcano et al.14 A 9:1 mixture of concentrated H2SO4/H3PO4 (360:40 mL) was slowly added to graphite flakes (1.0 g) in a 2 L plastic container; then KMnO4 (25.0 g) was very slowly added under slow stirring (at this stage slow stirring does not break GO flakes). The container temperature was about 38−42 °C. The reaction mixture was kept at about 40 °C without stirring overnight (16 h). After cooling (4−5 h), 1 L of cold DI water with 50 mL of 35% H2O2 was added (without stirring). The reaction mixture was kept overnight at room temperature to ensure complete oxidation of graphite powders by KMnO4 and H2O2. The green slurry was centrifuged at 4000 rpm for 5 min. 1.0 L of 1.0 M H2SO4 was added. The slurry was stirred with a Teflon rod and then centrifuged again at 4000 rpm for 5 min: two different layers were observed. The bottom layer consists of large graphite particles, and a top layer is composed of exfoliated graphene oxide sheets. After addition of 500 mL of DI water and centrifugation at 9000 rpm for 20 min (two times), the brown top layer was separated and dispersed into 2 L of DI water. The remaining large graphite particles were separated by low-speed centrifugation (4000 rpm for 3 min). The final GO gel (1−5 wt %) was formed by centrifugation (6000−9000 rpm for 1.5 h). The aqueous GO slurry was poured onto a polished Teflon Received: March 23, 2016 Revised: May 15, 2016 Published: May 16, 2016 12559
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The Journal of Physical Chemistry C plate (30.5 cm × 30.5 cm). About 10 layers of Scotch tape (3M) or paper were used as spacers to achieve the desired membrane thickness. The slurry was then spread using a glass bar. The sample was dried overnight, and the film was carefully lifted off. Supported membranes were prepared by drop-casting a small amount of the GO slurry onto a hydrophobic Si (100) wafer. Water evaporation from the slurry took place at ambient conditions over several hours. UV Irradiation. The GO membranes were placed under a UV light source (GE G8T5 with a peak wavelength of 254 nm and about 7−8 mW power output at this wavelength) inside a nonreflective enclosure. The distance between the light source and the samples was 18 cm. To prevent the unsupported membranes from curling up, they were either held in place by small weights along the edges or taped straight onto e.g. the FTIR sample holder. Samples were irradiated intermittently in 4 h intervals or continuously (specifics are indicated in the text and figure legends). Materials Characterization. Scanning electron microscopy (SEM) to study the surface topology of the GO membranes was performed using a JEOL JSM-5900 microscope. Revolution 2.40 software was used to record the images. Powder X-ray diffraction (XRD) data were recorded using a Rigaku Miniflex II powder diffractometer with monochromatic Cu Kα radiation (λ = 1.540 59 Å) and Bragg−Brentano geometry. The GO membrane samples were fixed onto quartz sample holders for the measurements. Samples that were irradiated intermittently were fixed onto the quartz holder before the first irradiation interval to ensure a reproducible sample configuration during the XRD measurements. Spectroscopic Analysis. UV/vis spectra were recorded over the range from 250 to 3300 nm with a UV−Vis−NIR Cary 5000 absorption spectrometer from Agilent Technologies. The data were recorded as transmittance spectra using the instrument’s software (Varian Cary WinUV). All spectra were recorded in double beam mode at a scan rate of 300 nm/min. The spectrum of an empty sample compartment was used as baseline for all measurements. Attenuated reflectance Fourier transform infrared (ATRFTIR) spectra in the range from 4000 to 600 cm−1 were recorded with a Nicolet iS10 (Thermo Scientific). The spectra shown were recorded at a resolution of 0.25 cm−1, employing a diamond Smart ITR accessory with a diamond crystal window (incident angle 42°, refractive index n = 2.4). Transmittance FTIR spectra over a range from 4000 to 400 cm−1 were obtained using a Thermo Nicolet Nexus FT-IR spectrometer equipped with a DTGS detector and a XT-KBr beam splitter. These spectra were recorded at a resolution of 1 cm−1. OMNIC spectra software was used for data acquisition and analysis for all FTIR spectra. GO membranes were mounted with carbon tape for X-ray photoelectron spectroscopy (XPS) measurements. High energy resolution data were collected utilizing a monochromated Al Kα X-ray source (λ = 1486.6 eV) and a GammaData/Scienta SES-200 hemispherical analyzer. The energy resolution of the SES-200 spectrometer is approximately 0.5 eV for the photoemission spectra reported here. No sample cleaning was performed prior to data collection. When necessary, a low energy electron flood gun was employed for charge compensation. Raman spectra were excited in a backscattering geometry using 8 mW of 532 nm radiation from an Ventus mpc6000 solid state diode laser. Scattered light was focused onto the slits
of a Spex Model 500 M single grating spectrometer (Jobin Yvon, Inc.) and detected using a Spec-10:100BR/XP backilluminated, deep-depletion CCD detector (Princeton Instruments). Slit widths were set at 100 μm, which provided a nominal resolution of 1.15 cm−1. Data acquisition and analysis was performed using Winspec software (Princeton Instruments) and Grams/32 Al. Spectroscopic ellipsometry data were collected with a J.A. Woollam V-VASE variable angle spectroscopic ellipsometer at angles of 70°, 75°, and 80° on a GO film supported on a Si(100) wafer. Below ∼1.5 eV, thickness interference fringes were observed. These could not be fit with a high degree of confidence but indicated that the GO film thickness in the measured region was approximately 2−3 μm. The best-fit thickness for each sample run (the supported film was not measured in exactly the same position each time) was used in subsequent fitting of the higher energy region (>1.5 eV), but it was observed that altering the thickness in this region did not affect the fit. This indicates that the GO film was sufficiently absorbing that it could be treated as infinitely thick. Ellipsometric data were fit using the VASE software from J.A. Woollam with a multistep process: first, a Cauchy fit which included absorption (k) was attempted. This fit was then used as the initial guess for an independent fit of n and k over the spectral region (point-by-point fit). The resulting complex dielectric function parameters ε1 and ε2 were then modeled with Sellmeier dispersion and Lorentzian oscillators, respectively, to enforce Kramers−Kronig consistency. The absorption coefficient, α, was calculated from α = 4πk/λ. Unsupported GO membranes were found to be too rough to obtain reliable ellipsometric data.
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RESULTS AND DISCUSSION Immediately after casting, 3−5 μm thick unsupported GO membranes are transparent and light brown in color. Figure 1a shows the top view of the surface of a cast GO membrane. The topology of cast GO membranes is relatively stable. For example, the texture of the membrane does not change significantly during 24 h exposure to UV light (Figure 1b). In comparison to what has been observed for GO membranes prepared by vacuum filtration,3 the top surface of the cast, large-area membranes is slightly rougher, but they display a very similar structure of layered stacks of GO flakes in crosssectional scanning electron micrographs (Figure 1c and Figure S1). The stacks of GO flakes are several hundred nanometers thick and extend several square micrometers in the plane of the GO membrane. If the GO membranes are stored under ambient conditions, they darken over the course of several weeks (Figure 2a). Thermally reducing GO to graphene requires temperatures of at least 125 °C;15 therefore, the observed change in our membranes is more likely photoinitiated. UV-light-induced reduction of aqueous GO suspensions has been reported previously.16,17 This led us to investigate the effect of UV light (254 nm) on freshly prepared GO membranes. UV exposure of the unsupported GO membranes was observed to cause the membranes to curl toward the UV light source significantly. In 2013, Giardi et al.18 reported an average UV light penetration depth of 10−15 μm into water-based GO/acrylic nanocomposite inks. The mechanical deformation of our GO membranes, however, indicates that any chemical or physical changes do not take place homogeneously over the entire membrane thickness. In contrast to the water-based GO/ 12560
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membrane (approximately 2−3 μm thick) are illustrated in Figure 3a−d. When the sample was initially stored under ambient conditions (not directly exposed to fluorescent room lighting), only a slight increase in index of refraction (Figure 3a) and absorbance (Figure 3b) were recorded over time. Exposing the sample to UV light for 24 h drastically increased both the index of refraction and the absorption coefficient of the GO membrane (Figure 3a,b), similar to the observed change for unsupported membranes. Comparing the index of refraction with recent DFT calculations19 of the optical properties of fully and partially oxidized GO, good qualitative agreement is seen between the refractive index for the as-cast membrane and the calculated index for fully oxidized GO; the change in the refractive index after UV exposure is consistent with the calculated index for partially reduced GO. The increase of the extinction coefficient with UV exposure (Figure S2) also follows the calculated general trend19 of increasing extinction with GO reduction. The π-plasmon peak of the absorption coefficient shifts from 223 nm (5.55 eV) to ∼237 nm (5.23 eV). A red-shift of the π-plasmon peak has been observed for aqueous GO suspensions during hydrazine reduction to graphene.17,20 On the basis of ab initio density functional theory (DFT) calculations, Johari and Shenoy predicted a redshift of the π-plasmon peak with decreasing concentrations of epoxide and hydroxyl groups in GO.21 These results strengthen our hypothesis that UV-light-induced GO reduction can also occur in dry GO membranes, not only in suspended GO flakes.16 The bandgaps calculated from the ellipsometry data of the supported GO membrane confirm the trend observed in UV/ vis absorption measurements of the unsupported membranes. As shown in the Tauc plots in Figures 3c,d and the resulting bandgap values in Table 1, the as-cast membrane exhibits an indirect bandgap of 1.61 eV. (For consistency, all indirect bandgap values were determined by extrapolating a line fit to the data from 3.2 to 3.7 eV; however, to better compare to reported values,16,22 a second extrapolation at lower photon energy is also reported in parentheses in Table 1.) As the membrane is slowly reduced while stored in ambient conditions, then dramatically reduced during 24 h UV exposure, the indirect bandgap decreases to 0.53 eV. The higher energy direct bandgap follows the same trend, reducing from 4.55 to 3.62 eV. Both theoretical calculations23,24 and experimental results16,22 indicate that GO should possess an indirect bandgap. As GO is reduced toward graphene, the bandgap reduces and eventually crosses over from indirect to direct.23 The bandgap reduction in Table 1 is thus consistent with a reduction of the supported GO membrane with UV exposure. Yeh et al.16 reported a bandgap reduction for an aqueous solution of GO flakes exposed to UV light, although the indirect bandgap values listed in Table 1 are lower than those observed by Yeh et al. (2.4−3.0 eV for GO suspension; 1.4−1.5 eV after 6 h irradiation). Likewise, Mathkar et al.22 observed an indirect bandgap value of ∼1 eV after approximately 100 h of exposure to hydrazine vapor. Although we did not observe a crossover from an indirect to direct bandgap, close inspection of Figure 3c reveals a significant absorption tail at lower photon energy after UV exposure, but no clearly linear region could be identified. Further UV exposure for an additional 48 h did not significantly change the position of the π-plasmon peak or the bandgap characteristics of the supported membrane.
Figure 1. Scanning electron micrographs depicting the top view of an unsupported GO membrane (a) as-cast and (b) after exposure to UV light for 24 h; (c) side view of an as-cast unsupported GO membrane (with thickness measurements indicated in green and yellow).
acrylic nanocomposite inks,18 our membranes are composed entirely of GO flakes, which absorb UV light more strongly than the GO/polymer mixture. It is therefore not surprising that the UV light does not penetrate as deeply into the material, and we observe a gradual, thickness-dependent transformation which macroscopically leads to stress and curvature of the membrane. Because the extent of the UV-light-induced reduction therefore depends on the thickness of the GO membrane as well as the exposure time and irradiation power, we have studied the GO membranes with bulk as well as surface-sensitive spectroscopic analysis techniques and also prepared samples in a variety of different ways. Figure 2b shows the steady decrease in the transparency of an unsupported GO membrane with increasing UV exposure time. A red-shift of the absorption edge is also detectable (3.35−2.85 eV). GO membranes darken visibly with UV light exposure. The experimental setup depicted in Figure 2c illustrates rudimentary DUV photolithography, which was performed using metallic letters to shade parts of the membrane. Using precisely machined photolithography masks, patterning on the submillimeter scale can be achieved over a large area, providing an advantage over laser-initiated DUV lithography and other techniques. To study the changes in the bandgap of the material with greater accuracy, we prepared a supported GO membrane on a Si (100) wafer and subsequently exposed it to UV light. The results of ellipsometry measurements on this supported 12561
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Figure 2. (a) Transmission-mode UV/vis absorption spectra of an unsupported as-cast GO membrane right after preparation (black), after 47 days (red), and after approximately 16 months (blue) having been stored under ambient conditions. (b) Evolution of transmission-mode UV/vis absorption spectra of an unsupported GO membrane with increasing UV light exposure. (c) Photograph of an unsupported GO membrane after UV exposure; the lighter regions were masked with metallic letters (shown) during the illumination period.
1627 cm−1. No significant change in the ratio between bound and unbound water molecules (ν(OH) ∼ 3260 cm−1 and ν(OH) ∼ 3395 cm−1, respectively)25 during UV exposure is detectable. This confirms that any d-spacing changes observed by powder XRD are indeed caused by a reduction of GO to a more graphene-like material and not from the removal of unbound water molecules between the GO layers. This in turn means: First, the observed mechanical stress along the GO membrane top surface cannot be attributed to a drying process and associated shrinkage of the top surface under UV irradiation. Instead, it provides an indication that the GO reduction does not occur uniformly throughout the membrane thickness. Second, if the water content remains constant under UV light illumination, we can conclude from the FTIR spectra in Figure 5a that the content of the basal hydroxyl groups decreases over time. Basal hydroxyl groups exhibit a stretching vibration, ν(OH), at 3620 cm−1.26 In our spectra a shoulder is visible at 3595 cm−1, which coincides with the high frequency end of the envelope of vibrations for the differently bound water molecules (ν(OH) ∼ 3260 cm−1 and ν(OH) ∼ 3395 cm−1, respectively). Considering the dominant vibrations from water content in the membranes and the absence of any other vibration mode around 3600 cm−1, we attribute this shoulder to the absorption of the basal hydroxyl ν(OH) mode, in agreement with literature reported assignments. Based on
Powder X-ray diffraction (XRD) patterns of supported and unsupported 2D-hierarchically stacked GO membranes exhibit a decrease in d-spacing with increasing UV exposure time. For an unsupported GO membrane that was irradiated intermittently it decreased from 8.9 to 8.5 Å (Figure 4). Possibly this shift could arise from removal of water molecules intercalated between GO sheets; however, the data can also be interpreted as indicative of GO reduction. Our observed d-spacings are well within the range of reported values for GO (d = 6.1−12 Å).2 The intensity decrease, peak broadening, and change in peak shape seen in the XRD pattern are a reflection of increasing inhomogeneity along the z-direction of the sample (thickness). Especially the (001) peak observed after 24 h UV light exposure shows increased peak broadening, but on the basis of deconvolution attempts, we believe that no actual peak splitting occurs. In general, we expect the observed decrease in d-spacing to be more pronounced closer to the membrane surface that was directly exposed to UV light. The unsupported membranes are sufficiently transparent to allow through-transmittance Fourier transform infrared spectroscopy (FTIR) measurements before and after UV exposure. Because the overall membrane transparency decreases over the entire range of the FTIR spectra (from 400 to 4000 cm−1) during UV exposure, the relative transmittance of all spectra in Figure 5a was normalized next to a very weak vibration mode at 12562
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Figure 4. Powder X-ray diffraction patterns illustrating a shift of the (001) peak to higher angle which indicates a decrease of the d-spacing between GO layers of an unsupported GO membrane with increasing UV light exposure.
spectra of the as-cast GO membrane and the same membrane after exposure to UV light for 24 h (shown in Figure 5a), the ratio between the basal hydroxyl groups and the unbound water decreases from 0.41:1 to 0.27:1. This observation supports our other findings that GO gets reduced to a more graphene-like material under UV light irradiation. Additionally, distinct differences are observable between the through-transmittance FTIR spectra of the as-cast GO membrane and the UV-irradiated material: an increase of absorption intensity around 1590 cm−1 and decrease in absorption intensity at ∼1050 cm−1 (Figure 5a). As indicated above, the degree of reduction from GO to a more graphenelike material is not uniform over the thickness of the membrane, and these FTIR spectra represent an average of the chemical composition of the GO membrane in the zdirection. Therefore, we do not attempt to assign specific chemical changes to these differences in absorption intensity beyond stating that these areas around 1590 cm−1 (increasing absorption) and ∼1050 cm−1 (decreasing absorption) are indicative of changes in ν(CC) and ν(C−C) or ν(C−O) intensities, respectively. We subsequently conducted attenuated total reflectance (ATR) FTIR on our unsupported GO membrane samples. ATR-FTIR is a surface analysis technique. The penetration depth of the IR beam depends on the specific wavelength λ, the incident angle of the IR beam, and the refractive indices of the ATR crystal and the sample. For our measurements the penetration depth of the IR beam is ∼2.5 μm. This allows us to investigate any differences in vibrational modes with higher certainty that they originate from only one or a related group of compositional changes in the GO membrane. As shown in Figure 5b, the exposed (blue spectra) and the unexposed (red spectra) surface of the unsupported GO membrane exhibit differences in relative vibration mode intensities compared to the as-cast GO membrane. Furthermore, it becomes evident that the nonirradiated side of the membrane more closely resembles the spectra of an as-cast GO membrane. Given the
Figure 3. (a) Index of refraction, n, from ellipsometric measurements of a supported GO film on an Si(100) wafer. (b) Absorption coefficient derived from spectroscopic ellipsometry measurements of a supported GO membrane on Si(100) wafer. Extrapolation of linear regions to zero are shown as dashed lines in Tauc plots to estimate direct (c) and indirect (d) bandgap.
Table 1. Direct and Indirect Bandgaps Determined by Ellipsometry GO membrane as-cast 74 days ambient 24 h UV exposure 72 h UV exposure
direct bandgap (eV)
indirect bandgap (eV)
4.55 4.49 3.62 3.63
3.13 (1.61) 2.96 (1.50) 0.53 0.52
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Figure 5. (a) Evolution of transmittance-mode FTIR spectra of an unsupported GO membrane with increasing UV light exposure; arrows are to guide the eye indicating the changes in relative vibration mode intensities at ∼1590 and ∼1050 cm−1. (b) ATR-FTIR spectra comparing an as-cast GO membrane to the spectra of the UV light irradiated and nonirradiated surfaces of a GO membrane (28 h exposure) and (c) transmittance-mode FTIR spectra of an as-cast GO membrane and a GO membrane exposed to UV light from both sides for 24 h each.
average 3−5 μm thickness of our membranes, this confirms that the UV light penetration depth in our experiments is limited to approximately 2.5 μm at most. This is corroborated by the ellipsometry measurements, which indicate that the changes in optical properties of the supported membrane occurred uniformly throughout the membrane depth (approximately 2−3 μm). Upon UV light exposure, the vibration mode at 1041 cm−1 in the ATR-FTIR spectra decreases in intensity relative to the vibration modes at ∼1160 and ∼1215 cm−1 (sh). The absorption intensity ratio between the modes at ∼1215 (sh), ∼1160, and 1041 cm−1 respectively changes from 1:1.12:1.28 for the as-cast membrane (black spectra) to 1:1.13:1.24 for the nonirradiated surface of the GO membrane (red spectra) and 1:1.08:1.07 for the irradiated surface of the GO membrane. Additionally, the vibration mode at ∼1600 cm−1 shifts to lower wavenumbers (from 1623 to 1605 cm−1) and becomes more pronounced compared to the neighboring mode at 1733 cm−1
(Figure 5b). Even if variations in the chemical composition of the GO sample could be eliminated completely, assigning precise vibration modes in the ATR-FTIR spectra below 1900 cm−1 would be difficult because of overlapping stretching, bending, and other deformation modes. The following assignment is based on combined experimental and theoretical (cluster-based first-principles calculations) results by Acik et al.:26 The vibration mode at 1041 cm−1 in all likelihood is either derived from the presence of edge hydroxyl groups or dioxolane species (6-membered ring containing two nonadjacent O atoms; see Table S1 in the Supporting Information). Considering the red-shift we observed for the π-plasmon peak, in conjunction with the relative weakening of the vibration mode at 1041 cm−1, a reduction of hydroxyl groups along the edges of the GO flakes as origin of both phenomena seems plausible, especially since Ganguly et al. found that the edge carboxyl groups on suspended GO flakes are highly unstable during thermal reaction.27 Pyranone (6-membered 12564
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The Journal of Physical Chemistry C ring containing one O atom adjacent to a CO moiety) or pyran-like ether (6-membered ring containing one O atom) species could give rise to the absorbance at ∼1160 cm−1. Peroxide (6-membered ring containing two adjacent O atoms) or furan-like ether (5-membered ring containing one O atom) moieties give rise to vibrations at frequencies around 1265− 1280 cm−1.26 Although we cannot say with complete certainty what the origin of these different vibrational frequencies is in our spectra, a reduction of dioxolane species might lead to an increase in pyranone as well as pyran- and furan-like ether moieties, which would explain the observed changes in the FTIR spectra between 1500 and 1000 cm−1. Modes due to in-plane vibration of sp2-hybridized CC bonds have been reported for reduced graphene oxide and graphene around 1600−1500 cm−1.28 Upon UV light exposure GO membrane samples consistently display an increase in absorbance for a vibration mode at ∼1600 cm−1. In the through-transmittance FTIR spectra (Figure 5a) this vibration band initially is only visible as a shoulder on the much stronger ν(CO) band at 1737 cm−1, and again its increase in intensity is accompanied by a shift toward slightly lower wavenumbers. That the ATR-FTIR measurement picks up a stronger ν(C C) for the as-cast GO membrane confirms that the same chemical changes, which are observed for GO membranes at an accelerated rate under UV light exposure, indeed occur more slowly under ambient conditions (Figure 5b). These changes on the GO membrane surface, possibly confined to several tenths of nanometer, dominate the surface sensitive ATR-FTIR spectra. Nevertheless, the increased reduction of the membrane material under UV light exposure is unmistakable (Figure 5b), and the data correlate well with the changes in vibrational mode intensities observed by through-transmittance FTIR for the same GO membrane (Figure 5a). The UV-light-exposed top surface and the nonirradiated bottom part of the GO membrane can be separated by soaking the membrane in water overnight. ATR-FTIR measurements of the separated GO layers confirm the described observations (Figure S3). Exposing our 3−5 μm thick GO membrane to UV light from both sides for 24 h each yields a membrane that has undergone chemical and physical changes throughout the entire stack of GO flakes. This improves the suitability of through-transmittance FTIR measurements to determine any differences between the two membranes. All changes in relative absorption intensity between the different vibration modes, which we discussed above, become very pronounced when comparing this GO membrane to the starting material (Figure 5c): The relative intensity between the ν(CO) band at ∼1735 cm−1 and the ν(CC) band, which shifts from 1627 to 1583 cm−1, changes from a 1:0.45 to a 1:0.88 ratio. The absorption intensity ratio between the modes at 1165 and 1051 cm−1 respectively changes from 1:0.96 to 1:0.67. The slight shifts in vibration mode locations are further indications that the membranes irradiated on one side only do not display a homogeneous chemical composition. The mechanism of reduction of the GO membranes after exposure to UV light for 24 h on each side can be further probed by XPS. The limited escape depth of photoelectrons means that XPS is only sensitive to the top 5−10 nm of the GO membrane, which in this case is advantageous since the membrane surface receives the highest UV intensity during exposure. As shown in Figure 6a and Table 2, after UV exposure the C/O ratio calculated from the survey spectra (Figures S4 and S5) increases from 1.87 to 2.88, and significant
Figure 6. High resolution C 1s (a) and O 1s (b) XPS spectra of GO membranes before (as-cast) and after UV exposure on each side for 24 h. Peaks were fit with Voigt functions after subtraction of a Shirley background. Boxed values in (a) are the ratio of the total C and O peak areas calculated from survey spectra and corrected for photoionization cross section.35
changes in both the C 1s and O 1s regions are observed, as compared to the as-cast membrane. For the C 1s data, the lowest binding energy (BE) peak is typically assigned to CC, and peaks at increasingly higher BE are assigned to C−OH, CO, and OC−OH.29−33 Despite this apparent simplicity, the precise BE shifts from the CC peak to the C−O moieties are not agreed upon in the literature.34 Further, epoxide C−O− C is difficult to distinguish in C 1s spectra, likely appearing with the C−OH contribution, but the possibility exists that it is shifted such that it overlaps with the CO BE.31 The peak assignments in the present work were made following ref 33 and are indicated in Figure 6a, with details of the fitted peaks given in Table 2. Charge compensation was necessary when measuring the as-cast GO membrane, but not when measuring the UV-exposed membrane. In neither case have the peak positions been shifted. However, the close correspondence in peak position for the CC peaks from each membrane (see Table 2) indicates that the charge compensation placed the spectra for the as-cast membrane near the intrinsic BE values; 12565
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The Journal of Physical Chemistry C Table 2. Results from XPS Peak Fittinga as-cast GO peak assignment
position (eV)
fraction of total area
UV 24 h each side fraction of O species area
position (eV)
fraction of total area
fraction of O species area
0.91 0.09
